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July 2000
ML4866* 3.3V Output DC-DC Step-Down Converter
GENERAL DESCRIPTION
The ML4866 is a high efficiency pulse width modulated (PWM) buck regulator designed for use in 5V systems or portable equipment that need a compact, efficienct 3.3V supply. It has a switching frequency of 120kHz and uses synchronous rectification to optimize power conversion efficiency. Unlike other solutions, the ML4866 requires no external diodes or FETs. The ML4866 can provide up to 500mA of output current, and operates over an input voltage range of 3.5V to 6.5V (3 to 4 cells or a 5 VDC supply). A complete switched mode power converter can be quickly and easily implemented with few external components. Thanks to a built-in autoburst mode, power conversion efficiency of this DC-DC converter can exceed 90% over more than 2 decades of output load current. Stability and fast loop response are provided by current programming and a current sense circuit. The ML4866 also has a SHDN pin for use in systems which have power management control. Undervoltage lockout and soft start are also built in.
FEATURES
s
High power conversion efficiency over 2 decades of load current No external FETs or diodes; minimum external components 3.5V to 6.5V input voltage range Significantly extends battery life over linear regulator based solutions Micropower operation Low shutdown mode quiescent current
s
s s
s s
(* Indicates Part is End Of Life as of July 1, 2000)
BLOCK DIAGRAM
5 VIN CURRENT SENSE
7 VL
1 VOUT
UVLO/ SHUTDOWN OSC
BUCK CONTROL
REFERENCE
SLOPE COMPENSATION
-
ERROR AMPLIFIER
- +
BURST
+
VREF
SHDN 6 3
VREF
BURST 4 2
COMP 8
GND
1
ML4866
PIN CONFIGURATION
ML4866 8-Pin SOIC (S08)
VOUT COMP VREF BURST 1 2 3 4 TOP VIEW 8 7 6 5 GND VL SHDN VIN
PIN DESCRIPTION
PIN NAME FUNCTION PIN NAME FUNCTION
1 2 3 4
VOUT COMP V REF BURST
Regulated 3.3V output Connection point for an external compensation network 1.25V reference output This pin controls when the control circuit switches between PWM and PFM modes of operation
5 6 7 8
V IN SHDN VL GND
Input voltage Pulling this pin low shuts down the regulator Buck inductor connection Ground
2
ML4866
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. VIN ................................................................................................... 7V Voltage on any other pin ......... GND - 0.3V to VIN + 0.3V Peak Switch Current (IPEAK) ......................................... 2A Average Switch Current (IAVG) ..................................... 1A Junction Temperature .............................................. 150C Storage Temperature Range ....................... -65C to 150C Lead Temperature (Soldering 10 Sec.) ..................... 260C Thermal Resistance (qJA) .................................... 160C/W
OPERATING CONDITIONS
Temperature Range ML4866CS ................................................. 0C to 70C ML4866ES .............................................. -20C to 70C ML4866IS ............................................... -40C to 85C VIN Operating Range ................................... 3.5V to 6.5V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = 5V, L = 50H, COUT = 100F, RCOMP = 390kW, CCOMP = 15nF, TA = Operating Temperature Range (Note 1)
SYMBOL REFERENCE
VREF Output Voltage 0 < I(VREF) < -5A, IOUT = 0mA 1.22 1.25 1.27 V
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PWM REGULATOR
fOSC Oscillator Initial Accuracy Oscillator Total Variation Soft Start VIN to VOUT Delay BURST Burst Mode Threshold BURST PWM Mode Threshold BURST Bias Current Output Voltage IOUT = 200mA IOUT = 20mA, BURST = 0V Line Regulation Load Regulation VIN = 4V to 6.5V, TA = 25C IOUT = 100mA to 500mA, TA = 25C IOUT = 5mA to 100mA, BURST = 0V, TA = 25C Temperature Stability Total Variation TA = -40C to 85C Line, Load, Temp 3.2 3.28 3.3 3.38 250 IOUT = 200mA, TA = 25C Line and Temp 100 90 115 130 3 400 500 850 35 3.4 3.48 2 2.5 2.5 1 5 165 185 5 kHz kHz ms mV mV A V V % % % % %
SHUTDOWN
UVLO Startup Threshold UVLO Shutdown Threshold SHDN Threshold SHDN Bias Current 3.2 2.9 2 -5 3.5 3.1 V V V A
3
ML4866
ELECTRICAL CHARACTERISTICS
SYMBOL SUPPLY
IIN VIN Current IOUT = 0mA, BURST = 5V IOUT = 0mA, BURST = 0V SHDN = 0V
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
(Continued)
CONDITIONS MIN TYP MAX UNITS
PARAMETER
400 120 20
500 220 35
A A A
FUNCTIONAL DESCRIPTION
The ML4866 is a current-mode, step-down (buck) converter designed to keep the buck inductor current in the continuous conduction mode (CCM). Current-mode operation provides faster output response to input voltage and output current changes along with cycle-by-cycle current limiting. CCM inductor current is preferred when the highest conversion efficiencies are required. For high efficiencies at low output current, the ML4866 contains an autoburst function which automatically switches from pulse width modulation (PWM) to pulsed frequency modulation (PFM) operation when the output current drops below 100mA. Selection of either mode is possible by applying the correct logic level signal to the BURST pin. When operating in PWM mode, loop compensation of the ML4866 is simplified due to its transconductance type error amplifier. An under voltage lockout (UVLO) circuit within the ML4866 enables the converter when the input voltage is greater than 3.25V and disables it when the input voltage is below 3.10V. The IC can also be disabled externally by applying a logic low signal to the SHDN pin. When disabled, the ML4866 draws less than 20A of current. The internal 1.25V bandgap reference is made available via the VREF pin, and may be used for general applications requiring less than 10A of current. For proper operation, this pin must always be bypassed to GND with a 100nF capacitor. ramping the inductor current down to 0mA. This action is repeated until the output voltage returns to its nominal setting and begins again when the output drops below its nominal setting. The rate or frequency at which this "bursting" occurs is directly proportional to the output current. When the average output current rises above 130mA, the ML4866 returns to PWM operation. For applications having a load current range of less than 100mA and greater than 130mA, the BURST pin should be left open and bypassed to ground with a 15nF or larger capacitor. It is possible to tailor an application for the highest possible efficiency by externally forcing the ML4866 into either control mode. Applying a logic low level to BURST forces the IC into PFM mode. Conversely, a logic high places it in PWM mode. Care should be taken to avoid reducing the efficiency by placing the controller in the least efficient mode for a given output current.
BURST MODE
Burst (PFM) mode is a method of regulating the output voltage by applying a variable frequency modulation technique to the buck inductor. This method maintains higher efficiencies at light loads than if PWM were used. If BURST is left open, the ML4866 switches from PWM mode to PFM mode when the output current falls below 100mA. When the output voltage falls out of regulation while in PFM mode, the internal buck switch turns on and ramps the inductor current up to 300mA. The buck switch then turns off and the synchronous switch turns on,
VIN - VOUT L -VOUT L I
Figure 1. Inductor Current
4
ML4866
DESIGN CONSIDERATIONS
INDUCTOR SELECTION
Figure 1 shows the inductor current in a step-down converter operating in CCM. Note that the inductor current does not reach zero during each switching cycle. This is unlike discontinuous conduction mode (DCM) where the inductor current is allowed to reach zero. CCM operation generally results in lower peak to peak output ripple voltage and higher circuit efficiencies because of lower peak and RMS currents in the switching FETs and buck inductor. The minimum value of inductance required for CCM operation with a 6.5V input and a load range of 100mA to 500mA is: L> L> VOUT ( VIN ( MAX) - VOUT ) 2 VIN ( MAX) IOUT
( MIN)
100 98 96
EFFICIENCY (%)
IOUT = 100mA 94 92 90 88 86 IOUT = 10mA
IOUT = 500mA 3.5 4.0 4.5 5.0 5.5 6.0 6.5
INPUT VOLTAGE (V)
fSW
(1)
3.3V (6.5V - 33V) . > 68mH 2 6.5V 100mA 120kHz
Figure 2. Efficiency vs. Input Voltage
To guarantee reliable operation, the peak inductor current must be between 80% and 85% of its maximum rated value. This value is the sum of the inductor peak to peak current and the maximum output current:
IL(P -P) =
IL(P -P) =
A partial listing of inductor manufacturers with standard parts which meet the criteria for use with the ML4866 is given below. Coiltronics Dale Coilcraft XFMRS, Inc Sumida (561) 241-7876 (605) 665-9301 (847) 639-6400 (317) 834-1066 (847) 956-0666
2 VOUT
( MAX)
( VIN ( MIN) - VOUT
( MAX) )
VIN ( MIN) fSW ( MIN) L
2 3.465V ( 4.0V - 3.465V) = 103mA 4.0V 90kHz 100mH
( MAX )
(2)
CAPACITOR SELECTION
(3) A typical digital system requires a peak to peak output ripple voltage of no greater than 1% to 3% of the nominal output voltage. In a step-down converter, the largest contributor to ripple voltage is almost always the product of the inductor peak-to-peak current times the output capacitor's equivalent series resistance. To select the correct capacitor, first calculate the minimum capacitance value required: C OUT > VOUT ( VIN ( MAX) - VOUT ) 8 VP -P ( MAX) VIN ( MAX) L fSW 2 (4)
IL(PEAK) = IOUT
+
VOUT
( MAX )
( VIN ( MIN) - VOUT ( MAX) )
VIN ( MIN) fSW L
IL(PEAK) = IOUT ( MAX) +
3.465V ( 4.0V - 3.465V) = 550mA 4.0V 120kHz 100mH
For the highest efficiency, inductor core and copper losses must be minimized. Good high frequency core material such as Kool-Mu, ferrite or Molyperm are popular choices for this converter. Disregarding physical size requirements, the lowest loss inductor will generally be the one with the highest peak current rating. Figure 2 displays the efficiency of the ML4866 under various input voltage and output current conditions. These results were obtained using a Coiltronics CTX100-4 inductor having the following specifications: Nominal Inductance - 100H Peak Current Rating - 950mA DC Resistance - 175mW
C OUT >
. 3.3 (6.5V - 33V) 8 33mV 6.5V 100mH 120kHz 2
> 4.27mF
Next, calculate the maximum permissible ESR of the output capacitor:
ESR <
(0.033) < 0.33W (0.1)
(5)
When limited space is available, tantalum capacitors are the best choice. Electrolytic capacitors can be used and will be less expensive, but the ESR for low capacitance values as needed here will be much higher than for the same value tantalum. Table 2 lists the ESR values for a number of general purpose tantalum capacitors which are widely available from a number of sources. A 47F capacitor was chosen for the design example.
5
ML4866
DESIGN CONSIDERATIONS
FREQUENCY COMPENSATION
Frequency compensation of the ML4866 is required when the converter is operating in PWM mode. Two simple methods are provided to ensure the converter is frequency stable. Both these methods will work only if the inductor current is selected to be in CCM at the maximum load current (see Inductor Selection). The first, called dominant pole compensation, is used when non-varying loads are expected. This method requires a single capacitor connected from the error amplifier output (COMP Pin) to ground. For loads which change suddenly, the transient response (or bandwidth) of the circuit must be increased to prevent the output voltage from going outside of the regulation band. The method used to accomplish this is called zero/pole compensation and requires a series resistor capacitor network from COMP to ground. To determine which method works best for a given application, apply the components found from the zero/pole compensation method to an actual circuit and examine the output voltage variation. If the voltage variation is acceptable, connect the simpler, single capacitor and re-check the output voltage for acceptable load transient response. (Continued)
VARYING LOAD CURRENT
To minimize output voltage variations due to rapidly changing load currents, use the series RC zero compensation method to find the compensation network component values that will improve the output voltage response to load transients. The unity gain bandwidth of the converter is extended to 15kHz using an RC network determined by:
R COMP >
C COMP =
f G , where G = O gm fCOMP
1 50p R COMP
(7) (8)
Where f0 = 15kHz, fCOMP = 640Hz, RCOMP > 375kW (use 390kW, 5%), and CCOMP = 16nF (use 15nF). Either method of compensation for CCM mode with result in continued stability as the ML4866 changes to DCM mode at lighter load currents. Figure 3 shows a typical application circuit for the ML4866.
NON-VARYING LOAD CURRENT
For the best possible response to load transients using only a single capacitor, dominant pole compensation is implemented with a single capacitor value of:
C COMP = gm 2 fCOMP
(6)
Where fCOMP is the unity gain crossover point (640Hz), gm = 62.5mho, and CCOMP > 15.5nF (choose a standard 18nF or 22nF capacitor). The value of CCOMP can be increased but at the risk of increased output voltage variations with transient loads.
VOUT 3.3V
33F
CAPACITANCE
4.7F 10F 22F 33F 47F 100F
VOLTAGE RATING
16V 6.3V 16V 6.3V 10V 6.3V
SIZE
3216 3216 7343 6032 7343 7343
ESR @ 100kHz
0.490W 0.368W 0.149W 0.291W 0.144W 0.088W
15nF 100nF 390k
VOUT COMP VREF BURST
100H ML4866 1 2 3 4 8 7 6 5
GND VL SHDN VIN VIN 3.5V to 6.5V
15nF
100F
100nF
Table 2. ESR Values for Low Cost Tantalum Capacitors
Figure 3. Typical Application Circuit
6
ML4866
LAYOUT
For proper performance, all components should be placed as close to the ML4866 as possible. Particular attention should be paid to minimize the length of the connections between the COMP and VREF pins to GND. Also avoid bringing these traces and the associated components close to VL. It is always recommended that a 10F or greater capacitor be connected to VIN of the ML4866. A 33F tantalum capacitor and 100nF film or ceramic capacitor is recommended when powering the ML4866 from Lithium or Alkaline cells. Ground and power planes must be large enough to carry the current the converter has been designed to supply. A sample PC board layout is shown in Figure 4.
Figure 4. Sample PC Board Layout
7
ML4866
PHYSICAL DIMENSIONS
inches (millimeters)
Package: S08 8-Pin SOIC
0.189 - 0.199 (4.80 - 5.06)
8
PIN 1 ID
0.148 - 0.158 0.228 - 0.244 (3.76 - 4.01) (5.79 - 6.20)
1 0.017 - 0.027 (0.43 - 0.69) (4 PLACES)
0.050 BSC (1.27 BSC)
0.059 - 0.069 (1.49 - 1.75)
0 - 8
0.055 - 0.061 (1.40 - 1.55)
0.012 - 0.020 (0.30 - 0.51)
0.004 - 0.010 (0.10 - 0.26)
0.015 - 0.035 (0.38 - 0.89)
0.006 - 0.010 (0.15 - 0.26)
SEATING PLANE
ORDERING INFORMATION
PART NUMBER
ML4866CS (End Of Life) ML4866ES (EOL) ML4866IS (Obsolete)
TEMPERATURE RANGE
0C to 70C -20C to 70C -40C to 85C
PACKAGE
8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08)
DS4866-01
(c) Micro Linear 1997. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners. Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874. Japan: 2,598,946; 2,619,299. Other patents are pending. Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel before deciding on a particular application.
2092 Concourse Drive San Jose, CA 95131 Tel: (408) 433-5200 Fax: (408) 432-0295 www.microlinear.com
9/8/97 Printed in U.S.A.
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