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High Performance Wide Input Range Dual Synchronous Buck Controller
POWER MANAGEMENT Description
The SC2545 is a high performance dual PWM controller. It is designed to convert a wide ranged input voltage down to two independent output rails. The SC2545 support "peel off " tracking for the two outputs at start up which means the two outputs ramp up together till the one with lower output level reaching the regulation. The PWM operations of the two channels are 180 degree out of phase which can greatly reduce the size and the cost of the input capacitors. Synchronous Buck PWM topology and voltage mode control allow high efficiency operation, fast transient responses, and flexible component selection for easy designs. A 10V internal linear regulator provides the bias for the controller, and this voltage is optimized for gate drivers to deliver high efficiency. The power sequencing is fully supported including independent start up, and power good output. In the shut down mode the controller only draws 100nA from the supply. The controller also offers full protection features for the conditions of under voltage, over voltage, and the over current. There is no need for a current sensing resistor because the MOSFET on resistance is used for the sensing element. The switching frequency is adjustable from 100 kHz to 300 kHz. Two packages TSSOP-24 and MLPQ-24 are offered.
SC2545
Features
Independent dual-outputs "Peel off "start up trackingIndependent dual-outputs Wide input voltage range: 4.5V~28V Adjustable output voltage down to 0.75V Flexible power sequencing with enable and power good output Synchronous Buck topology with voltage mode control Out of phase operation to reduce cost of input capacitor 10V internal regulator for gate driver to deliver high efficiency Programmable switching frequency: 100kHz~300kHz Full protection: UVLO, OVP and programmable OCP No need for current sense resistor Low shutdown current (100nA typical) 24 lead TSSOP and MLPQ packages Fully WEEE and RoHS Compliant
Applications
Systems with 4.5V~28V input LCDTV and PDPTV Network and telecom systems
Typical Application Circuit
SC2545
VIN+ VCC 2 ENABLE FB1 3 4 5 6 VIN+ 7 8 VO1 9 10 11 FB1 12 VO1 1 VIN VCC EN FB1 ERROUT1 SS1 ILIM1 BST1 DRVH1 PHASE1 DRVL1 PGND AGND ROSC PWRGD FB2 ERROUT2 SS2 ILIM2 BST2 DRVH2 PHASE2 DRVL2 PVCC 24 23 22 21 20 19 18 17 16 15 14 13 VCC FB2 VO2 VIN+ PWGRD FB2
Pinout shown as TSSOP-24.
Revision: August 10, 2005 1 www.semtech.com
SC2545
POWER MANAGEMENT Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied.
Parameter BST1, BST2 to PGND VIN to PGND ILIM1, ILIM2 and EN to PGND VC C and PVC C to PGND PGND to AGND BST1 to PH1, BST2 to PH2, D RVH1 to PH1, D RVH2 to PH2 D RVL1, D RVL2 to PGND PHASE, D RVL to AGND pulse (100nS), peak voltage All Other Pi ns to AGND Storage T mperature Range e Juncti on T mperature e Lead T mperature (Solderi ng) 10 Sec for TSSOP-24 e Lead T mperature (IR Reflow) for MLPQ-24 e Thermal Resi stance Juncti on to C ase TSSOP-24 MLP-24 Thermal Resi stance Juncti on to Ambi ent TSSOP-24 MLP-24 ESD Rati ng (Human Body Model)
Symbol V b st Vin
Maximum 38 28 VIN 14 +/- 0.3 -0.3 to 14 -0.3 to VC C -3 -0.3 to VC C
U nits V V V V V V V V V
o o
TSTG TJ TLEAD JC
-60 to +150 -40 to +150 260 23 2 78 25 TBD
o
C C C
o
C /W
JA ESD
o
C /W kV
Note: This device is ESD sensitive. Use of standard ESD handling precautions is required.
Electrical Characteristics
Unless specified: TA = 25 C, VIN=16V, Fs=200kHz .
o
Parameter U ndervoltage Lockout Start Threshold UVLO Hysteresi s Pow er Supply Operati ng C urrent (I IN-IPVCC) Vcc Regulated Vcc Load Regulati on Level Vcc Li ne Regulati on PVcc Operati ng C urrent D i sable Qui escent C urrent Main Sw itcher output Li ne Regulati on Load Regulati on Output Voltage Accuracy
Test C onditions
Min
Typ
Max
U nit
Vcc ri si ng Vcc falli ng 200
4.5
V mV
SS1/SS2/EN =Hi gh, FS=200kHz VIN > 12V I_load=0~20mA Vi n=12~24V 200KHz, 1nF on HG, 1nF on LG EN=low
6.0 10 2 2 14 0.1
10
mA V % % mA
10
uA
5V0.5 0.5 0.750 0.765
% % V
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SC2545
POWER MANAGEMENT Electrical Characteristics (Cont.)
o
Unless specified: TA = 25 C, VIIN=16V, Fs=200kHz
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SC2545
POWER MANAGEMENT Electrical Characteristics (Cont.)
Unless specified: TA = 25 C, VIN=16V, Fs=200kHz
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Ordering Information
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Notes: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead free package. Device is fully WEEE and RoHS compiant.
Pin Configurations
Top View
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24 PIN TSSOP
MLPQ-24
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SC2545
POWER MANAGEMENT Pin Descriptions for SC2545TSTRT (TSSOP)
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SC2545
POWER MANAGEMENT Pin Descriptions for SC2545MLTRT (MLPQ)
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SC2545
POWER MANAGEMENT Block Diagram (One PWM channel
shown)
PVCC BST
Protect 1
DRVH PHASE
ROSC
Oscillator Ramp generator
Ramp1
3
+ PWM 1 S R Q
PHASE1 PVCC
PVCC DRVL
2 CLK1
-
Protect 1
ERROUT FB FB 0.75V SS + OUT R R + OUT OUT Clamp S UVLO OCP OUT + VCC VIN VCC OVP OUT ENABLE 0.89V + FB1 /Q R Protect 1 10u A ILIM VCC E/A 0.65V +
-
EN
ON/OFF VCC UVLO
0.75V Band Gap 1 10V LDO Band Gap 2 0.75V 0.675V FB1 FB2 + OUT OVP OCP
PWRGD
AGND
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SC2545
POWER MANAGEMENT Applications Information
Overview The SC2545 is a constant frequency 2-phase voltage mode step-down PWM switching controller driving all Nchannel MOSFETs. The two channels of the controller operate at 180 degree out of phase from each other. Since input currents are interleaved in a two-phase converter, input ripple current is lower and smaller input capacitance can be used for filtering. Also, with lower inductor current and smaller inductor ripple current per phase, overall I2 R losses are reduced. requency Setting F req uency Se tting The frequency of the SC2545 is user- programmable. The oscillator of SC2545 can be programmed with an external resistor from the Rosc pin to the ground. The step-down controller is capable of operating up to 300kHz. The relationship between oscillation frequency versus oscillation resistor is shown in Figure 1. The advantages of using constant frequency operation are simple passive component selection and ease of feedback compensation. Before setting the operating frequency, the following trade-offs should be considered. 1) Passive component size 2) Circuitry efficiency 3) EMI condition 4) Minimum switch on time 5) Maximum duty ratio For a given output power, the sizes of the passive components are inversely proportional to the switching frequency, whereas MOSFETs/Diodes switching losses are proportional to the operating frequency. Other issues such as heat dissipation, packaging and cost issues are also to be considered. The frequency bands for signal transmission should be avoided because of EM interference.
350 300 Fsw(KHz) 250 200 150 100 50 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180
Sof t Star t and "Peel of f "Tracking P T During start-up, the reference voltage of the error amplifier equals 30% of the voltage on Css (soft-start capacitor) which is connected between the SS pin and ground. When the controller is enabled (by pulling EN pin high), one internal 84uA current source, ISS, (soft start current) will charge the soft-start capacitor gradually. The PWM output starts pulsing when the soft start voltage reaches 1V. This soft start scheme will ensure the duty cycle to increase slowly, therefore limiting the charging current into the output capacitor and also ensuring the inductor does not saturate. The soft start capacitor will eventually be charged up to 2.5V. The soft-start sequence is initiated when EN pin is high and Vcc >4.5V or during recovery from a fault condition ( OCP, OVP, or UVLO). The period of start up can be programed by the soft start capacitor:
Tss =
Css x 2.5V 84 A
By tieing the two soft start pins together, the two output rails track each other at start up till the rail with lower set level reaching regulation. Please refer the waveform on page 20. Shutdown When the EN pin is pulled low, an internal 15uA current source discharges the soft-start capacitor and DRVH/ DRVL signals stop pulsing. The output voltage ramps down at a rate determined by the load condition. The SC2545 can also be shutdown by pulling down directly on the SS pin. The designer needs to consider the slope of the SS pin voltage and choose a suitable pull down resistor to prevent the output from undershooting. Shutdown can also be triggered when an OCP condition occurs. When an OCP condition is detected, DRVH and DRVL will stop pulsing and enter a "tri-state shutdown" with the output voltage ramping down at a rate determined by the load condition. The internal 15uA current source will begin discharging the soft-start capacitor and when the soft-start voltage reaches 0.65V, DRVL will go high.
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Rosc(KOHM)
Figure 1. Switching frequency versus Rosc.
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SC2545
POWER MANAGEMENT Applications Information (Cont.)
Over Current Protection (OCP) The inductor current is sensed by using the low side MOSFET Rds(on) . After low side MOSFET is turned on, the OCP comparator starts monitoring the voltage drop across the MOSFET. The OCP trip level is programmed by the resistor from the ILIM pin to the phase node. There is an internal current source that flows out of the ILIM pin which will generate a voltage drop on the setting resistor. When the sum of the setting resistor voltage and the MOSFET drain to source voltage is less then zero, the OCP condition will be flagged. This functionality is depicted in Figure 2. The following formula is used to set the OCP level
TG IL
100nS Blanking
OCP Active
Figure 3. OCP comparator timing chart. Voltage (UVLO) U nder Voltage Lock Out (UVLO) The UVLO circuitry monitors Vcc and the soft start begins once Vcc ramps up above 4.5V. There is a built in 200mV hysteresis for the UVLO ramp down threshold. The gate driver output will be in "tri-state" (both high side and low side MOSFET off) once Vcc ramps down bellow 4.2V (typical), and the soft start cap will be discharged by internal 15uA current sink. Over Voltage Pro (OVP) Ov er V oltage Pr o t ection (O VP) The OVP circuitry monitors the feedback voltages, If either feedback voltage exceeds 0.89V, the OVP condition is registered. Under this condition, the DRVH pins will be pulled low, and the DRVL pins will be pulled high. This will create a "crow bar" condition for the input power rail in case the high side MOSFET is failed short. The crow bar operation may trip the input supply to prevent the load from seeing more voltage. Power Good Output
10 A x RILIM = I L _ PEAK x RDS ( ON )
When OCP is tripped, both high side and low side MOSFETs will be turned off and this condition is latched. At the same time, the soft start cap will be discharged by the internal current source of 15uA. When the Vss drops bellow 0.65V, the DRVL pin will go high again. To avoid switching noise during the phase node commutation, a 100nS blanking time is built in after the low side MOSFET is turned on, as shown in Fig. 3.
VCC
10uA
DRVH ILIM DRVL
OUTPUT
OCP Out
+
-
Figure 2. Block diagram of over current protection.
The power good is an open collector output. The PWRGD pin is pulled low at start up if any of the two feedback voltages below 90% of its regulation level. The ramp down threshold of the signal is 80% of the regulation target. External pull up is required for the PWRGD pin, and the pull up resistor should be chosen such that the pin does not sink more than 2mA when PWRGD is low.
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SC2545
POWER MANAGEMENT Applications Information (Cont.)
General Design Procedure for a Step-down Power Converter Selection criterias and design procedures for the following parameters are described: 1) Output inductor (L) type and value 2) Output capacitor (Co) type and value 3) Input capacitor (Cin) type and value 4) Power MOSFETs 5) Current sensing and limiting circuit 6) Voltage sensing circuit 7) Loop compensation network The following step-down converter specifications are needed: Input voltage range: Vin,min and Vin,max Input voltage ripple (peak-to-peak): DVin Output voltage: Vo Output voltage accuracy: e Output voltage ripple (peak-to-peak): DVo Nominal output (load) current: Io Maximum output current limit: Io,max Output (load) current transient slew rate: dIo (A/ s) Circuit efficiency: K Inductor (L) and Ripple Current Both step-down controllers in the SC2545 operate in synchronous continuous-conduction mode (CCM) regardless of the output load level. The output inductor selection/design is based on the output DC and transient requirements. Both output current and voltage ripples are reduced with larger inductance but it takes longer to change the inductor current during load transients. Conversely smaller inductance results in lower DC copper losses but the AC core losses (flux swing) and the winding AC resistance losses are higher. A compromise is to choose the inductance such that peak-to-peak inductor ripple-current is 20% to 30% of the rated output load current. Assuming that the inductor current ripple (peak-to-peak) value is *Io, the inductance value will then be G
/ 9R ' G,R IV
,/UPV
,R
G
The followings are to be considered when choosing inductors. a) Inductor core material: For higher efficiency applications above 300 kHz, ferrite, Kool-Mu and polypermalloy materials should be used. Low-cost powdered iron cores can be used for cost sensitiveapplications below 300 kHz but with attendant higher core losses. b) Select inductance value: Sometimes the calculated inductance value is not available off-the-shelf. The designer can choose the adjacent (larger) standard inductance value. The inductance varies with temperature and DC current. It is a good engineering practice to re-evaluate the resultant current ripple at the rated DC output current. c) Current rating: The saturation current of the inductor should be at least 1.5 times of the peak inductor current under all conditions. Output Capacitor (C o) and V out Ripple The output capacitor provides output current filtering in steady state and serves as a reservoir during load transient. The output capacitor can be modeled as an ideal capacitor in series with its parasitic ESR and ESL as shown in Figure 4.
&R
/HVO
5HVU
Figure 4. An equivalent circuit of output. If the current through the branch is i b(t), the voltage across the terminals will then be
The peak current in the inductor becomes (1+ /2)*Io
YR W 9R
GL W LE WGW /HVO E 5HVULE W GW &R
W
and the RMS current is
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This basic equation illustrates the effects of ESR, ESL, and Co on the output voltage.
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SC2545
POWER MANAGEMENT Applications Information (Cont.)
The first term is the DC voltage across Co at time t=0. The second term is the voltage variation caused by the charge balance between the load and the converter output. The third term is voltage ripple due to ESL and the fourth term is the voltage ripple due to ESR. The total output voltage ripple is then a vector sum of the last three terms. Since the inductor current is a triangular waveform with peak-to-peak value *Io, the ripple-voltage caused by inductor current ripples is The voltage rating of aluminum capacitors should be at least 1.5Vo. The RMS current ripple rating should also be greater than
Io 23 .
Usually it is necessary to have several capacitors of the same type in parallel to satisfy the ESR requirement. The voltage ripple caused by the capacitor charge/ discharge should be an order of magnitude smaller than the voltage ripple caused by the ESR. To guarantee this, the capacitance should satisfy
Co > 10 . 2fsR esr
vC
Io , 8Cofs
Io , D
the ripple-voltage due to ESL is
v ESL = L esl f s
and the ESR ripple-voltage is
In many applications, several low ESR ceramic capacitors are added in parallel with the aluminum capacitors in order to further reduce ESR and improve high frequency decoupling. Because the values of capacitance and ESR are usually different in ceramic and aluminum capacitors, the following remarks are made to clarify some practical issues. Remark 1: High frequency ceramic capacitors may not carry most of the ripple current. It also depends on the capacitor value. Only when the capacitor value is set properly, the effect of ceramic capacitor low ESR starts to be significant. For example, if a 10 F, 4m ceramic capacitor is connected in parallel with 2x1500 F, 90m electrolytic capacitors, the ripple current in the ceramic capacitor is only about 42% of the current in the electrolytic capacitors at the ripple frequency. If a 100 F, 2m ceramic capacitor is used, the ripple current in the ceramic capacitor will be about 4.2 times of that in the electrolytic capacitors. When two 100 F, 2m ceramic capacitors are used, the current ratio increases to 8.3. In this case most of the ripple current flows in the ceramic decoupling capacitor. The ESR of the ceramic capacitors will then determine the output ripple-voltage. Remark 2: The total equivalent capacitance of the filter bank is not simply the sum of all the paralleled capacitors. The total equivalent ESR is not simply the parallel combination of all the individual ESR's either. Instead they should be calculated using the following formula.
Ceq( ) = (R1a + R1b )2 2C1a C1b + (C1a +C1b )2 2 2 (R1a C1a + R1b C1b )2C1aC1b + (C1a + C1b )
2 2
vESR = ResrIo .
Aluminum capacitors (e.g. electrolytic) have high capacitances and low ESLs. The ESR has the dominant effect on the output ripple voltage. It is therefore very important to minimize the ESR. Other types to choose are solid OS-CON, POSCAP, and tantalum. When determining the ESR value, both the steady state ripple-voltage and the dynamic load transient need to be considered. To meet the steady state output ripplevoltage spec, the ESR should satisfy
Re sr1
VO IO
To limit the dynamic output voltage overshoot/ undershoot within a (say 3%) of the steady state output voltage from no load to full load, the ESR value should satisfy
Re sr 2
3%VO IO
Then, the required ESR value of the output capacitors should be Resr = min{Resr1,Resr2 }.
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SC2545
POWER MANAGEMENT Applications Information (Cont.)
R ( ) : Ceqeq ()== R1aR1b (R1a + R1b )2 C1a C1b + (R1b C1b + R1a C1a )
2 2 2 2
(R1a + R1b ) C1a C1b + (C1a + C1b )
2 2 2 2
2
where R 1a and C 1a are the ESR and capacitance of electrolytic capacitors, and R1b and C1b are the ESR and capacitance of the ceramic capacitors, respectively (Figure 5).
In Figure 6 the DC input voltage source has an internal impedance Rin and the input capacitor Cin has an ESR of Resr. MOSFET and input capacitor current waveforms, ESR voltage ripple and input voltage ripple are shown in Figure 7.
i Q1
C1a C1b Ceq
i Cin
R1a R1b Req
Vesr
Figure 5. Equivalent RC branch. Req and Ceq are both functions of frequency. For rigorous design, the equivalent ESR should be evaluated at the ripple frequency for voltage ripple calculation when both ceramic and electrolytic capacitors are used. If R1a = R1b = R1 and C1a = C1b = C1, then Req and Ceq will be frequencyindependent and Req = 1/2 R1 and Ceq = 2C1. Input Capacitor (Cin) The input supply to the converter usually comes from a pre-regulator. Since the input supply is not ideal, input capacitors are needed to filter the current pulses at the switching frequency. A simple buck converter is shown in Figure 6.
2 1 L1 Q1 Rin Resr 2
VCin
Figure 7. Typical waveforms at converter input. It can be seen that the current in the input capacitor pulses with high di/dt. Capacitors with low ESL should be used. It is also important to place the input capacitor close to the MOSFETs on the PC board to reduce trace inductances around the pulse current loop. The RMS value of the capacitor current is approximately
ICin = Io D[(1 + 2 D D )(1 - )2 + 2 (1 - D) ]. 12
The power dissipated in the input capacitors is then PCin = ICin2Resr. For reliable operation, the maximum power dissipation in the capacitors should not result in more than 10oC of temperature rise. Many manufacturers specify the maximum allowable ripple current (ARMS) rating of the capacitor at a given ripple frequency and ambient temperature. The input capacitance should be high enough to handle the ripple current. It is common pratice that multiple capacitors are placed in parallel to increase the ripple current handling capability.
D1 Co VDC Cin 1 Ro
Figure 6. A simple model for the converter input.
(c) 2005 Semtech Corp. 12 www.semtech.com
SC2545
POWER MANAGEMENT Applications Information (Cont.)
Sometimes meeting tight input voltage ripple specifications may require the use of larger input capacitance. At full load, the peak-to-peak input voltage ripple due to the ESR is
'Y (65 G 5 HVU ,R ',R &LQ IV
The peak-to-peak input voltage ripple due to the capacitor is
'Y & |
Choosing Power MOSFETs Main considerations in selecting the MOSFETs are power dissipation, MOSFETs cost, and packaging. Switching losses and conduction losses of the MOSFETs are directly related to the total gate charge (Cg) and channel on-resistance (Rds(on)). In order to judge the performance of MOSFETs, the product of the total gate charge and on-resistance is used as a figure of merit (FOM). Transistors with the same FOM follow the same curve in Figure 8.
From these two expressions, CIN can be found to meet the input voltage ripple specification. In a multi-phase converter, channel interleaving can be used to reduce ripple. The two step-down channels of the SC2545 operate at 180 degrees from each other. If both stepdown channels in the SC2545 are connected to the same input rail, the input RMS currents will be reduced. Ripple cancellation effect of interleaving allows the use of smaller input capacitors. When two channels with a common input are interleaved, the total DC input current is simply the sum of the individual DC input currents. The combined input current waveform depends on duty ratio and the output current waveform. Assuming that the output current ripple is small, the following formula can be used to estimate the RMS value of the ripple current in the input capacitor. Let the duty ratio and output current of Channel 1 and Channel 2 be D1, D2 and Io1, Io2, respectively. If D1<0.5 and D2<0.5, then
,&LQ | ',R ' ,R

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Figure 8. Figure of Merit curves. The closer the curve is to the origin, the lower is the FOM. This means lower switching loss or lower conduction loss or both. It may be difficult to find MOSFETs with both low Cg and low Rds(on. Usually a trade-off between Rds(on and Cg has to be made. MOSFET selection also depends on applications. In many applications, either switching loss or conduction loss dominates for a particular MOSFET. For synchronous buck converters with high input to output voltage ratios, the top MOSFET is hard switched but conducts with very low duty cycle. The bottom switch conducts at high duty cycle but switches at near zero voltage. For such applications, MOSFETs with low Cg are used for the top switch and MOSFETs with low Rds(on) are used for the bottom switch. MOSFET power dissipation consists of a) conduction loss due to the channel resistance Rds(on); b) switching loss due to the switch rise time tr and fall time tf; and c) the gate loss due to the gate resistance RG.
If D1>0.5 and (D1-0.5) < D2<0.5, then
,&LQ | ,R ' ,R ,R ' ' ,R

If D1>0.5 and D2 < (D1-0.5) < 0.5, then
,&LQ | ,R ' ,R ,R ' ' ,R

If D1>0.5 and D2 > 0.5, then
,&LQ | ' ' ,R ,R ' ,R ' ,R
a 2005 Semtech Corp. 13 www.semtech.com
SC2545
POWER MANAGEMENT Applications Information (Cont.)
Switch Top Switc h The RMS value of the top switch current is calculated as
The conduction losses are then Ptc = IQ1,rms2 Rds(on).
In Figure 9, Qgs1 is the gate charge needed to bring the gate-to-source voltage Vgs to the threshold voltage Vgs_th. Qgs2 is the additional gate charge required for the switch current to reach its full-scale value I ds, and Q gd is the . charge needed to charge gate-to-drain (Miller) capacitance when Vds is falling. Switching losses occur during the time interval [t1, t3]. Defining tr = t3-t1 and tr can be approximated as
tr = (Q gs 2 + Q gd )R gt Vcc - Vgsp .
Rds(on) varies with temperature and gate-source voltage. Curves showing R ds(on) variations can be found in manufacturers' data sheet. From the Si4860 datasheet, Rds(on) is less than 8m when V gs is greater than 10V. However R ds(on) increases by 50% as the junction temperature increases from 25oC to 110oC. The switching losses can be estimated using the simple formula
1 Pts = 2 ( t r + t f )(1 + 2 )Io Vin fs .
where Rgt is the total resistance from the driver supply rail to the gate of the MOSFET. It includes the gate driver internal impedance Rgi, external resistance Rge and the gate resistance Rg within the MOSFET : Rgt = Rgi+Rge+Rg. Vgsp is the Miller plateau voltage shown in Figure 9. Similarly an approximate expression for tf is
tf = (Q gs 2 + Q gd )R gt Vgsp .
where t r is the rise time and t f is the fall time of the switching process. Different manufactures have different definitions and test conditions for t and t . To clarify r f these, we sketch the typical MOSFET switching characteristics under clamped inductive mode in Figure 9.
Vds Volts Ids Miller plateau Vgs
Only a portion of the total losses Pg = QgVccfs is dissipated in the MOSFET package. Here Qg is the total gate charge specified in the datasheet. The power dissipated within the MOSFET package is
Rg R gt
Ptg =
Q g Vcc fs .
Vgs th
The total power loss of the top switch is then Pt = Ptc+Pts+Ptg.
Qgs1 Qgs2 t0 t1 t2
Qgd t3 Gate charge
Figure 9. MOSFET switching characteristics
If the input supply of the power converter varies over a wide range, then it will be necessary to weigh the relative importance of conduction and switching losses. This is because conduction losses are inversely proportional to the input voltage. Switching loss however increases with the input voltage. The total power loss of MOSFET should be calculated and compared for high-line and low-line cases. The worst case is then used for thermal design.
(c) 2005 Semtech Corp.
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SC2545
POWER MANAGEMENT Applications Information (Cont.)
Bottom Switch The RMS current in bottom switch is given by
G ,4UPV ,R '
The conduction losses are then Pbc=IQ2,rms2 Rds(on). where Rds(on) is the channel resistance of bottom MOSFET. If the input voltage to output voltage ratio is high (e.g. Vin=12V, Vo=1.5V), the duty ratio D will be small. Since the bottom switch conducts with duty ratio (1-D), the corresponding conduction losses can be quite high. Due to non-overlapping conduction between the top and the bottom MOSFETs, the internal body diode or the external Schottky diode across the drain and source terminals always conducts prior to the turn on of the bottom MOSFET. The bottom MOSFET switches on with only a diode voltage between its drain and source terminals. The switching loss is negligible due to near zerovoltage switching. The gate losses are estimated as
Main Control Loop Design The goal of compensation is to shape the frequency response charatericstics of the buck converter to achieve a better DC accuracy and a faster transient response for the output voltage, while maintaining the loop stability. The block diagram in Figure 10 represents the control loop of a buck converter designed with the SC2545. The control loop consists of a compensator, a PWM modulator, and an LC filter. The LC filter and PWM modulator represent the small signal model of the buck converter operating at fixed switching frequency. The transfer function of the model is given by:
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The total bottom switch losses are then Pb=Pbc+Pbg. Once the power losses for the top and bottom MOSFETs are known, thermal and package design at component and system level should be done to verify that the maximum die junction temperature (Tj,max, usually 125oC) is not exceeded under the worst-case condition. The equivalent thermal impedance from junction to ambient ( T ja) should satisfy Fig. 10. Block diagram of the control loop. where VIN is the input voltage, Vm is the amplitude of the internal ramp, and R is the equivalent load. The model is a second order system with a finite DC gain, a complex pole pair at Fo, and an ESR zero at Fz, as shown in Figure 11. The locations of the poles and zero are determined by:
T ja depends on the die to substrate bonding, packaging material, the thermal contact surface, thermal compound property, the available effective heat sink area, and the air flow condition (natual or forced convection). Actual temperature measurement of the prototype should be carried out to verify the thermal design.
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a 2005 Semtech Corp.
15
SC2545
POWER MANAGEMENT Applications Information (Cont.)
The compensator in Figure 10 includes an error amplifier and impedance networks Zf and Zs. It is implemented by the circuit in Figure 12. The compensator provides an integrator, double poles, and double zeros. As shown in Figure 11, the integrator is used to boost the gain at low frequency. Two zeros are introduced to compensate excessive phase lag at the loop gain crossover due to the integrator (-90deg) and the complex pole pair (180deg). Two high frequency poles are designed to compensate the ESR zero and to attenuate high frequency noise.
A resistive divider is used to program the output voltage. The top resistor Rtop of the divider in Fig. 12 can be chosen from 20k : to 30k : . Then the bottom resistor Rbot is found from:
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Fig. 11. Bode plots for control loop design.
where 0.75V is the internal reference voltage of the SC2545. The other components of the compensator can be calculated using following design procedure: (1). Plot the converter gain, including LC filter and PWM modulator. (2). Select the open loop crossover frequency Fc located at 10% to 20% of the switching frequency. At Fc, find the required DC gain. (3). Use the first compensator pole Fp1 to cancel the ESR zero Fz. (4). Have the second compensator pole Fp2 at half the switching frequency to attenuate the switching ripple and high frequency noise. (5). Place the first compensator zero Fz1 at or below 50% of the power stage resonant frequency Fo. (6). Place the second compensator zero Fz2 at or below the power stage resonant frequency Fo. A MathCAD program is available upon request for the calculation of the compensation parameters.
& & 5 & 5
5WRS
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Fig. 12. Compensation network.
a 2005 Semtech Corp.
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SC2545
POWER MANAGEMENT Applications Information (Cont.)
PC Board Layout Issues Circuit board layout is very important for the proper operation of high frequency switching power converters. A power ground plane is required to reduce ground bounces. The followings are suggested for proper layout. Power Stage 1) Separate the power ground from the signal ground. In SC2545 design, use an isolated local ground plane for the controller and tie it to power grand. 2) Minimize the size of the high pulse current loop. Keep the top MOSFET, the bottom MOSFET and the input capacitors within a small area with short and wide traces. In addition to the aluminum energy storage capacitors, add multi-layer ceramic (MLC) capacitors from the input to the power ground to improve high frequency bypass. 3) Reduce high frequency voltage ringing. Widen and shorten the drain and source traces of the MOSFETs to reduce stray inductances. Add a small RC snubber if necessary to reduce the high frequency ringing at the phase node. Sometimes slowing down the gate drive signal also helps in reducing the high frequency ringing at the phase node if the EMI is a concern for the system. 4) Shorten the gate drive trace. Integrity of the gate drive (voltage level, leading and falling edges) is important for circuit operation and efficiency. Short and wide gate drive traces reduce trace inductances. Bond wire inductance is about 2~3nH. If the length of the PCB trace from the gate driver to the MOSFET gate is 1 inch, the trace inductance will be about 25nH. If the gate drive current is 2A with 10ns rise and falling times, the voltage drops across the bond wire and the PCB trace will be 0.6V and 5V respectively. This may slow down the switching transient of the MOSFETs. These inductances may also ring with the gate capacitance. 5) Put the decoupling capacitor for the gate drive power supplies (BST and PVCC) close to the IC and power ground.
Control Section 6) The frequency-setting resistor Rosc should be placed close to Pin 23. Trace length from this resistor to the analog ground should be minimized. 7) Place the bias decoupling capacitor right across the VCC and analog ground AGND.
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SC2545
POWER MANAGEMENT Typical Application Circuit SC2544MLTRT
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a 2005 Semtech Corp.
18
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SC2545
POWER MANAGEMENT Evaluation Board - Bill of Material SC2544MLTRT
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a 2005 Semtech Corp.
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SC2545
POWER MANAGEMENT Typical Characteristics
CH1: CH2: CH3: CH4: TG1 BG1 TG2 BG2
CH1:Vi CH2:Ven CH3:VSS CH4:Vo
Gate waveform
Shut down by pulling down EN pin voltage.
CH1:TG CH2:BG CH3:VSS CH4:Vo
CH3:Vo CH4:Io
Start up.
Transient response(0-5A).
Vss Vo1 Vo2
CH1:TG CH2:BG CH3:TG CH4:BG
"Peel off "Start up tracking by tieing SS1 and SS2 together.
(c) 2005 Semtech Corp. 20
Over current protection (5A/10mV)
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SC2545
POWER MANAGEMENT Typical Characteristics (Cont.)
Vin=8V 100% Fs (khz) 80% Efficient 60% 40% 20% 0% 0 1 2 3 4 5 6 Io(A) 7 8 9 10 Vin=16V 190 188 186 184 182 180 178 -40 -10 20 50 80 110
Temperature (Degree C)
Efficiency Curve for Vout=3.3V.
Freq. vs. Temp. ( Rosc=75kohm, Vin=16V).
350 300 Fsw(KHz) 250 200 150 100 50 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180
10.5 10.3 Vcc (V) 10.1 9.9 9.7 9.5 10 12 14 16 18 20 22 24 26 28 30 Vin (V)
Rosc(KOHM)
Operating frequency vs. Rosc.
Vcc vs. Vin ( Ta=25 Degree C).
9 8 Io(A) 7 6 5 4 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 RILIM(KOHM)
Vcc (V)
10.13 10.11 10.09 10.07 10.05 10.03 -40 -20 0 20 40 60 80 100 120 Tj (Degree C)
RILIM vs. OCP (Vi=12V).
Vcc vs. Temp.
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SC2545
POWER MANAGEMENT Typical Characteristics (Cont.)
7.0 6.5 Icc (mA) 6.0 5.5 5.0 8 10 12 14 16 18 Vin(V) 20 22 24 26 28
Icc vs. Vin (25 DegreeC).
DRVL min Ton(nS)
500 480 460 440 420 400 -40 -20 0 20 40 60 80 100 120 Tj (Degree C)
DL min Ton vs. Tj (Vin=16V).
100 Dead time (nS) 80 60 40 20 0
-40
-20
0
20
40
60
80
100
120
Tj (Degree C)
Dead time vs. Tj (Vin=16V, DH falling to DL rising).
(c) 2005 Semtech Corp.
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SC2545
POWER MANAGEMENT Outline Drawing - TSSOP-24
DIMENSIONS MILLIMETERS INCHES MIN NOM MAX MIN NOM MAX
.047 .002 .006 .042 .031 .007 .012 .003 .007 .303 .307 .311 .169 .173 .177 .252 BSC .026 BSC .018 .024 .030 (.039) 24 0 8 .004 .004 .008 1.20 0.05 0.15 0.80 1.05 0.19 0.30 0.09 0.20 7.70 7.80 7.90 4.30 4.40 4.50 6.40 BSC 0.65 BSC 0.45 0.60 0.75 (1.0) 24 0 8 0.10 0.10 0.20
A e N 2X E/2 E1 PIN 1 INDICATOR ccc C 2X N/2 TIPS 123 e/2 B E D
DIM
A A1 A2 b c D E1 E e L L1 N 01 aaa bbb ccc
aaa C SEATING PLANE
D A2 A
C bxN bbb
A1 C A-B D GAGE PLANE 0.25
H c
L (L1)
01
SIDE VIEW
SEE DETAIL
A
DETAIL
A
NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. 4. REFERENCE JEDEC STD MO-153, VARIATION AD.
Land Pattern - TSSOP-24
X
DIM
(C) G Z C G P X Y Z
DIMENSIONS INCHES MILLIMETERS
(.222) .161 .026 .016 .061 .283 (5.65) 4.10 0.65 0.40 1.55 7.20
Y P
NOTES: 1.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804
(c) 2005 Semtech Corp. 23 www.semtech.com
SC2545
POWER MANAGEMENT Outline Drawing - MLPQ-24 (4 x 4mm)
A D B
DIM
A A1 A2 b D D1 E E1 e L N aaa bbb
DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX
.031 .035 .040 .000 .001 .002 - (.008) .007 .010 .012 .151 .157 .163 .100 .106 .110 .151 .157 .163 .100 .106 .110 .020 BSC .011 .016 .020 24 .004 .004 0.80 0.90 1.00 0.00 0.02 0.05 - (0.20) 0.18 0.25 0.30 3.85 4.00 4.15 2.55 2.70 2.80 3.85 4.00 4.15 2.55 2.70 2.80 0.50 BSC 0.30 0.40 0.50 24 0.10 0.10
Top View
PIN 1 INDICATOR (LASER MARK)
E
A2 A aaa C A1 D1 LxN E/2 C SEATING PLANE
Bottom View
E1 2 1 N bxN e D/2 bbb CAB
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
Land Pattern - MLPQ-24 (4 x 4mm)
K
(C)
H
G
Z
DIM C G H K P X Y Z
DIMENSIONS INCHES MILLIMETERS (.155) (3.95) 3.10 .122 .106 2.70 .106 2.70 .021 0.50 .010 0.25 .033 0.85 .189 4.80
X P
NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804
(c) 2005 Semtech Corp.
24
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