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SP7601 29V High Voltage LED Driver
FEATURES Wide Input Voltage Range 4.5V - 29V 1.2MHz Constant Frequency Operation Low 0.2V Reference Voltage Adjustable Overcurrent Protection PWM Dimming and Power sequencing Control RoHS Compliant, Lead Free Packaging: Small 6-Pin TSOT Package DESCRIPTION The SP7601 is a high current DC to DC Switching Buck LED Driver that features low component count and high efficiency due to a very low 0.2V reference voltage. High 1.2MHz switching frequency allows use of very small external components. The SP7601 is compatible with PWM dimming up to a frequency of 1kHz at 10% Duty Cycle. TYPICAL APPLICATION CIRCUIT
M1 L1
VIN
Cin 1 Vin
Vo / LED +
up to 29V
2 Gate 6 LX D1 Rs
LED1
GND
C1 0.1uF 3
SP7601 LEDn
VDR GND D2 5 Dimming and Power sequencing FB 4 Rf b Rset
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SP7601 29V High Voltage LED Driver Page 1
(c) 2007 Exar Corporation
ABSOLUTE MAXIMUM RATINGS
These are stress ratings only, and functional operation of the device at these ratings or any other above those indicated in the operation sections of the specifications below is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability. Input Voltage....................................................-0.3V to 30V Lx.............................................................-2V to 30V FB.................................................................-0.3V to 5.5V Storage Temperature.............................-65 C to 150 C Junction Temperature...................................-40C to 125C Lead Temperature (Soldering, 10 sec)...................300 C ESD Rating..................1kV LX, 2kV all other nodes, HBM
ELECTRICAL SPECIFICATIONS
Specifications are for TAMB=TJ=25C, and those denoted by apply over the full operating range, -40C< Tj <125C. Unless otherwise specified: VIN = 4.5V to 29V, CIN = 4.7F.
PARAMETER UVLO Turn-On Threshold UVLO Turn-Off Threshold UVLO Hysteresis Operating Input Voltage Range Operating Input Voltage Range Operating VCC Current Standby VCC Current Reference Voltage Reference Voltage Switching Frequency Peak-to-peak ramp Voltage Minimum ON-Pulse Duration Minimum Duty Cycle Maximum Duty Cycle Gate Driver Turn-Off Resistance Gate Driver Pull-Down Resistance Gate Driver Pull-up Resistance VIN - VDR voltage difference Overcurrent Threshold LX pin Input Current OFF interval during hiccup SHDN Threshold SHDN Threshold Hysteresis
MIN 4.0 3.8
TYP 4.2 4.0 0.2
MAX 4.5 4.3
UNITS V V V
CONDITIONS 0C to 125C 0C to 125C
4.5 7 2 0.6 0.2 0.186 960 1200 Vin/10 40 100 50 4 3 4.5 300 25 0.8 350 33 50 1.0 100
29 29 3 1 0.214 1440 100 0 60 8 6 5.5 400 40 1.2
V V mA mA V V kHz V ns % % k V mV A ms V mV
0C to 125C
VFB=0.1V, gate not switching VFB=1.2V, gate not switching
Ramp clamped to <= 4.2V
Internal resistor between GATE and VIN VIN=12V, VFB=0.1V, Measure resistance between GATE and VDR VIN=12V, VFB=0.3V, Measure resistance between GATE and VIN
Measure VIN - VDR, VIN>7V Measure VIN - LX VLX = VIN
Apply voltage to FB
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SP7601 29V High Voltage LED Driver Page 2
(c) 2007 Exar Corporation
PIN DESCRIPTION PIN # 1 2 3 PIN NAME VIN GATE VDR DESCRIPTION Input power supply for the controller. Place the input decoupling capacitor as close as possible to this pin. Connect to the gate terminal of the external P-channel MOSFET. Power supply for the internal driver. This voltage is internally regulated to about 5V below VIN. Place the decoupling capacitor between VDR and VIN as close as possible to the IC. Regulator feedback input. A feedback resistor is connected between this pin and the LED cathode. A current setting resistor is connected between the cathode and ground of the LED. This pin can be also used for dimming control and/or Power Sequencing. Ground pin. This pin is used as a current limit input for the internal current limit comparator. Connect to the drain pin of the external MOSFET through an optional resistor. Internal threshold is pre-set to 350mV nominal and can be decreased by changing the external resistor based on the following formula: VTRSHLD = 350mV - 33A * R BLOCK DIAGRAM
5V VIN
4 5
FB GND
6
LX
VDR Oscillator Vin - 5V LDO
VIN 5V Internal LDO I = k x VIN
FAULT VREF
PWM Latch Reset Dominant S GATE
FB
+ + -
R
Error Amplifier
PWM Comparator VDR FAULT
FAULT ENBL UVLO FAULT Register S 4-Bit counter
-
LX
50ms delay
Overcurrent Comparator VIN - 0.35V
+ -
R R Set Dominant
POR
1V
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SP7601 29V High Voltage LED Driver Page 3
+
33uA
GND
(c) 2007 Exar Corporation
GENERAL OVERVIEW The SP7601 is a fixed frequency, Voltagemode, non-synchronous buck PWM controller optimized for driving LEDs. Constant LED current is achieved using resistor RSET as shown in the page 1 schematic. A low 0.2V reference voltage minimizes power dissipation in RSET. A tight reference voltage tolerance of +/-7%, over full operating conditions, helps accurately program the LED current. High switching frequency of 1.2MHz (nominal) reduces the size of passive components. Dimming and power sequencing is achieved using a logic-level PWM signal applied to the FB pin via a diode. Overcurrent protection (OCP) is based on the high-side MOSFET's RDS(ON) and is programmable via a resistor placed at the LX node. Programming the LED Current Use the following equation to program the LED current: the output current is to be set at 0.35A, then RSET = 0.57. If the output LED has a corresponding VF of 3.5V, then the SP7601 will step down the VIN to 3.5V. If two of these LEDs are placed in series, then SP7601 will step down the voltage to 7V. Superimposed on ILED current is a current ripple that is equal in magnitude to the inductor current ripple. Current ripple will be nominally set to 10% of ILED by proper sizing of the inductor. Note that throughout this datasheet ILED and IOUT will be used interchangeably. Dimming Signal A logic-level PWM signal applied through a small-signal diode to the feed-back (FB) pin can be used to control dimming of the LED. This signal turns the MOSFET gate drive on/off, thereby modulating the average current delivered to the LED. For turning the MOSFET off, the minimum signal amplitude at the FB node must be 1.2V; and to turn it back on it must fall below 0.7V. The frequency of the PWM signal can be up to 1kHz.
Rset =
0.2V ................................. (1) ILED
Where 0.2 is SP7601 reference voltage The output voltage will adjust as needed to ensure average ILED is supplied. For example, if
Figure1- 1kHz, 10% duty cycle dimming signal applied to SP7601, ch1:LX, ch2: dimming signal, ch4: ILED
Figure2- 1kHz, 80% duty cycle dimming signal applied to SP7601, ch1:LX, ch2: dimming signal, ch4: ILED
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SP7601 29V High Voltage LED Driver Page 4
(c) 2007 Exar Corporation
GENERAL OVERVIEW Modulator Operation and Power Sequencing The SP7601 has a unique modulator design which improves the device's ability to operate at very high duty cycle. While seamless in operation as the duty cycle is increasing (input voltage falling), when the duty cycle is decreased (input voltage rising), the user will observe the switching frequency increasing in distinct fractions of the switching frequency. If the device is operating at 100% duty cycle -- a unique advantage of using a p-channel pass device -- and then the input voltage is increased, the frequency will start at 300kHz, then 400kHz, 600kHz, and then finally 1.2MHz. The frequency will tend to increase to the next higher fraction once the duty cycle reaches 75 to 65 percent. This is the normal operation of the device and should be expected. There is no impact on the LED current accuracy. If PWM dimming is being used as the input voltage is increased, one will see the frequency increasing when the duty cycle is < 90%. When power is initially applied, the device will begin operating as if the input voltage is increasing and may start operation at one of the fractional operating frequencies. Many users will prefer to have the device start operating at the nominal operating frequency, thus it is recommended that VIN be applied after FB is set at the high state (>1.2V). The regulator is now in standby and once VIN has reached steady-state, then FB is transitioned from a high to a low state. The controller then starts operating at nominal frequency. Another benefit of using power sequencing for power up is that it ensures all internal circuitry is alive and fully operational before the device is required to regulate the current through the LEDs. Depending on the operating conditions, there may be short duration (<100s) current spikes during initial application of VIN. Figure 3 shows an example of the current spikes.
Figure 3- current spikes generated at startup when power sequencing is not used, ch1:VIN, ch2: IOUT
LEDs are capable of handling these short duration spikes as long as they are not operating at their thermal limit. Since the regulator was "Off" before power was applied, it is unlikely the LED is under any type of thermal stress. EXAR does not recommend using the SP7601 in applications where dimming of the LED is achieved by PWM'ing the actual input power as is common in automotive dimming applications. Buck Operation Without Output Capacitor In order to be able to apply the aforementioned dimming signal to the LED, the output filter capacitor that is normally used with a buck converter must be removed from the circuit. Thus the LED current ripple equals the inductor current ripple. As a rule of thumb, current ripple should be limited to 10% of ILED. Allowing for a higher current ripple, while staying within LED manufacturer ripple guidelines, will reduce inductance and possibly inductor size.
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SP7601 29V High Voltage LED Driver Page 5
(c) 2007 Exar Corporation
GENERAL OVERVIEW Overcurrent Programming Calculate the ripple current requirement from: Resistor Rs can be used to program Overcurrent Protection (OCP). Use equation 2 for calculating the Rs value:
Irip = Io x
Vo Vin
D (1 - D ) ..................... (4)
Rs =
0.35V - (1.5 x 1.05 x Iocp x Rds(on) ) .... (2) 33uA
Where D is converter duty cycle:
D=
Where Iocp is the programmed overcurrent and is generally set 50% above nominal output current. The maximum value of Rs that can be used for programming OCP is 3k. Inductor Selection Select the inductor L1 for inductance, IRMS and ISAT. Calculate inductance from
Ceramic capacitors are recommended for input filtering due to their low Equivalent Series Resistance (ESR), Equivalent Series Inductance (ESL) and small form factor. MOSFET selection Select the MOSFET M1 for ON resistance RDS(ON), voltage rating (BVDSS) and gate-todrain charge (QGD). The recommended MOSFET voltage rating for 12V and 24V applications is 30V and 40V respectively. As a rule of thumb QGD should be less than 10nC. RDS(ON) must be selected such that, when operating at peak current and junction temperature, the Overcurrent threshold of the SP7601 is not exceeded. Allowing 50% margin for temperature coefficient of RDS(ON), 5% margin for inductor current ripple and 50% margin for OCP, the following expression can be used:
L=
Vo x (Vin - Vo ) ............................ (3) Vin x f x IL
Where: VIN is converter input voltage Vo is converter output voltage. Since voltage across RSET is small, Vo approximately equals VF (for a string of series connected LEDs Vo equals total VF) IL is inductor current ripple (nominally set to 10% of Io) Inductor ISAT and IRMS must allow sufficient safeguard over output current Io. As a rule of thumb these parameters should be 50% higher than Io. Where high efficiency is required, a low DCR inductor should be used. Input capacitor selection Select the input capacitor for capacitance and ripple current rating. Use the capacitances listed in table 1 as a starting point and, if needed, increase CIN. Io (A) < 0.7 0.71 to 1.2 > 1.2 Cin (uF) 2.2 4.7 2 x 4.7
350mV Rds(ON ) ........... (5) 1.5 x 1.5 x 1.05 x Io
Within this constraint, selecting MOSFETs with lower RDS(ON) will reduce conduction losses at the expense of increased switching losses. If a choice of several MOSFETs is available, select the highest RDS(ON) MOSFET that meets the above criteria. Schottky Rectifier selection Select the Schottky D1 for Voltage rating VR and current rating If. Voltage rating should be selected using the same guidelines outlined for MOSFET voltage selection. Current rating can be calculated from:
Vo If 1 - x Io ........................... (6) Vin
Table1- CIN selection
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SP7601 29V High Voltage LED Driver Page 6
(c) 2007 Exar Corporation
GENERAL OVERVIEW Note that in applications where duty cycle is low, Schottky losses comprise a larger percentage of converter losses. In order to improve the efficiency in these applications choose a Schottky that meets the calculated current rating and has a lower VF. Feedback resistor RFB RFB is part of the SP7601 loop compensation network. Use a 30k RFB for VIN of 20V and larger. Use RFB of 60K for VIN less than 20V. Capacitor C1 This is the decoupling capacitor for the power supply to the internal driver. Use a 0.1F capacitor and place it as closely to the VDR and VIN pins as possible. 4. RDS(ON) is calculated from equation (5):
350mV RDS (ON ) = 0.42Ohm 1.5 x 1.5 x 1.05 x 0.35 A
BVDSS should be at least 30V and QGD should be less than 10nC. An ANPEC APM4427K with RDS(ON)=0.14 Ohm, BVDSS=30V and QGD=1.1nC can be used. 5. Schottky current rating IF can be calculated from (6):
If
3 . 3V 1 - x 0 . 35 A = 0 . 3 A 12 V
Voltage rating should be 30V. An SL03 rated at 30V/1.1A or equivalent can be used. 6. Calculate Rs from equation (2):
Design example- Design a drive circuit for a 0.35A LED with a 12V input voltage. Nominal LED voltage is 3.3V. 1. Calculate RSET from equation (1):
Rs =
0.35V - (1.5 x 1.05 x 0.525 A x 0.14Ohm ) = 7098Ohm 33uA
Use the maximum recommended Rs of 3k since the calculated value is larger.
M1, ANPEC APM4427K 30V, 0.14 Ohm
Rset =
0.2V = 0.57Ohm 0.35 A
VIN
L1, COILTRONICS LD1-560 56uH, 0.5Arms
2. Calculate inductor value L1 from (3):
12V
2.2uF 1 Vin
2 Gate 6 LX 3k D1, Vishay SL03 30V, 1.1A
3.3V x (12V - 3.3V ) L1 = = 57uH 12V x 1.2MHz x 0.035 A
A COILTRONICS LD-560 inductor rated at 56H, 0.5ARMS and 0.57ASAT can be used. 3. Select input capacitor: A 2.2F CIN is needed as shown in table 1. From equation (4), the ripple current rating of CIN is a fraction of 0.35A. A 2.2uF, 16V ceramic capacitor easily meets this requirement and offers low ESR and ESL.
GND
C1 0.1uF 3
LED1
SP7601
4
VDR GND 5
FB
60k 1N4148 Dimming and Power sequencing 0.57 Ohm
Figure 4. Circuit schematic for design example
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SP7601 29V High Voltage LED Driver Page 7
(c) 2007 Exar Corporation
PACKAGE: 6PIN TSOT
Sept25-07 Rev D
EXAR
SP7601 29V High Voltage LED Driver Page 8 (c) 2007 Exar Corporation
ORDERING INFORMATION
RoHS/ Theta JA Min Max MSL Level Lead C/W Temp C Temp C Free SP7601EK1-L -40 125 Yes 134 L1 @ 260C SP7601EK1-L/TR -40 125 Yes 134 L1 @ 260C Part Number SP7601_EB -40 125 Pack Type Canister Tape & Reel Quantity Any 2500 Package 6 Pin TSOT 6 Pin TSOT Board
Not Applicable to Evaluation Board
For further assistance: Email: WWW Support page: Application Notes: Sipexsupport@sipex.com http://www.sipex.com/content.aspx?p=support http://www.sipex.com/applicationNotes.aspx
EXAR Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained here in are only for illustration purposes and may vary depending upon a user's specific application. While the information in this publication has been carefully checked; no responsibility, however, is assumed for inaccuracies. EXAR Corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless EXAR Corporation receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; (c) potential liability of EXAR Corporation is adequately protected under the circumstances.
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SP7601 29V High Voltage LED Driver Page 9
(c) 2007 Exar Corporation

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