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July 2000
ML4771* High Current Boost Regulator
GENERAL DESCRIPTION
The ML4771 is a continuous conduction boost regulator designed for DC to DC conversion in multiple cell battery powered systems. Continuous conduction allows the regulator to maximize output current for a given inductor. The maximum switching frequency can exceed 200kHz, allowing the use of small, low cost inductors. The ML4771 is capable of start-up with input voltages as low as 1.8V, and the output voltage can be set anywhere between 3.0V and 5.5V by an external resistor divider connected to the SENSE pin. An integrated synchronous rectifier eliminates the need for an external Schottky diode and provides a lower forward voltage drop, resulting in higher conversion efficiency. In addition, low quiescent battery current and variable frequency operation result in high efficiency even at light loads. The ML4771 requires a minimum number of external components to build a very small regulator circuit capable of achieving conversion efficiencies exceeding 85%.
FEATURES
s
Guaranteed full load start-up and operation at 1.8V input Continuous conduction mode for high output current Very low supply current (20A output referenced) for micropower operation Pulse Frequency Modulation and Internal Synchronous Rectification for high efficiency Maximum switching frequency > 200kHz Minimum external components Low ON resistance internal switching FETs Adjustable output voltage (3.0V to 5.5V)
s s
s
s s s s
(* Indicates Part is End Of Life as of July 1, 2000)
BLOCK DIAGRAM
L1
1 VL1 VIN 2 *CIN
6 VL2
VOUT 5 VOUT
R1 VBAT
COUT
GND 3
BOOST CONTROL
SENSE 4 R2
*CFB
PWR GND 8
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ML4771
PIN CONFIGURATION
ML4771 8-Pin SOIC (S08)
VL1 VIN GND SENSE 1 2 3 4 TOP VIEW 8 7 6 5 PWR GND NC VL2 VOUT
PIN DESCRIPTION
PIN NAME FUNCTION PIN NAME FUNCTION
1 2 3 4
VL1 VIN GND SENSE
Boost inductor connection Battery input voltage Ground Programming pin for setting the output voltage
5 6 7 8
VOUT VL2 NC
Output of the boost regulator Boost inductor connection No connection
PWR GND Return for the NMOS output transistor
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ML4771
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. VOUT ........................................................................... 7V Voltage on any other pin ..... GND - 0.3V to VOUT + 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 ML4771CS ................................................. 0C to 70C ML4771ES ............................................. -20C to 70C VIN Operating Range ML4771CS .................................... 1.8V to VOUT - 0.2V ML4771ES .................................... 2.0V to VOUT - 0.2V VOUT Operating Range ................................. 3.0V to 5.5V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1).
SYMBOL SUPPLY IIN IOUT(Q) IL(Q) VIN Current VOUT Quiescent Current VL Quiescent Current VIN = VOUT - 0.2V 2 25 5 35 1 A A A PARAMETER CONDITIONS MIN TYP MAX UNITS
PFM REGULATOR IL Peak Current VSENSE VOUT SENSE Comparator Threshold Voltage Output Voltage Load Regulation See Figure 1, IOUT = 0 See Figure 1, VIN = 2.4V, IOUT = 220mA 1.2 2.52 4.95 4.85 1.4 2.57 5.05 4.95 1.7 2.62 5.15 5.15 A V V V
Note 1:
Limits are guaranteed by 100% testing, sampling or correlation with worst case test conditions.
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ML4771
20H (Sumida CD75)
ML4771 VL1 VIN 100F VIN GND SENSE PWR GND NC VL2 VOUT 200F 259k 268k IOUT VIN
Figure 1. Application Test Circuit.
IL 1 VIN 2 VL1 RSENSE
6 VL2
START-UP
SYNCHRONOUS RECTIFIER CONTROL
Q2 A2
+ -
VOUT 5
VOUT
R1
+ -
A3 BOOST CONTROL
+
SENSE 4
A1
Q1 ISET
-
2.57V R2
GND 3
PWR GND 8
Figure 2. PFM Regulator Block Diagram.
IL(MAX) IL ISET 0 VOUT VL2 0 Q1 ON Q2 OFF Q1 OFF Q2 ON
Figure 3. Inductor Current and Voltage Waveforms.
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ML4771
FUNCTIONAL DESCRIPTION
The ML4771 combines a unique form of current mode control with a synchronous rectifier to create a boost converter that can deliver high currents while maintaining high efficiency. Current mode control allows the use of a very small, high frequency inductor and output capacitor. Synchronous rectification replaces the conventional external Schottky diode with an on-chip PMOS FET to reduce losses and eliminate an external component. Also included on-chip are an NMOS switch and current sense resistor, further reducing the number of external components, which makes the ML4771 very easy to use. REGULATOR OPERATION The ML4771 is a variable frequency, current mode switching regulator. Its unique control scheme converts efficiently over more than three decades of load current. A block diagram of the boost converter is shown in Figure 2. Error amp A3 converts deviations in the desired output voltage to a small current, ISET. The inductor current is measured through a 50mW resistor which is amplified by A1. The boost control block matches the average inductor current to a multiple of the ISET current by switching Q1 on and off. The peak inductor current is limited by the controller to about 1.5A. At light loads, ISET will momentarily reach zero after an inductor discharge cycle , causing Q1 to stop switching. Depending on the load, this idle time can extend to tenths of seconds. While the circuit is not switching, only 20A of supply current is drawn from the output. This allows the part to remain efficient even when the load current drops below 200A. Amplifier A2 and the PMOS transistor Q2 work together to form a low drop diode. When transistor Q1 turns off, the current flowing in the inductor causes pin 6 to go high. As the voltage on VL2 rises above VOUT, amplifier A2 allows the PMOS transistor Q2 to turn on. In discontinuous operation, (where IL always returns to zero), A2 uses the resistive drop across the PMOS switch Q2 to sense zero inductor current and turns the PMOS switch off. In continuous operation, the PMOS turn off is independent of A2, and is determined by the boost control circuitry. Typical inductor current and voltage waveforms are shown in Figure 3.
DESIGN CONSIDERATIONS
OUTPUT CURRENT CAPABILITY The maximum current available at the output of the regulator is related to the maximum inductor current by the ratio of the input to output voltage and the full load efficiency. The maximum inductor current is approximately 1.25A and the full load efficiency may be as low as 70%. The maximum output current can be determined by using the typical performance curves shown in Figures 4 and 5, or by calculation using the following equation:
IOUT( MAX) = 125 .
V V
IN( MIN) OUT
0.7A
(1)
INDUCTOR SELECTION The ML4771 is able to operate over a wide range of inductor values. A value of 10H is a good choice, but any value between 5H and 33H is acceptable. As the inductor value is changed the control circuitry will automatically adjust to keep the inductor current under control. Choosing an inductance value of less than 10H will reduce the component's footprint, but the efficiency and maximum output current may drop. It is important to use an inductor that is rated to handle 1.5A peak currents without saturating. Also look for an inductor with low winding resistance. A good rule of thumb is to allow 5 to 10mW of resistance for each H of inductance. The final selection of the inductor will be based on tradeoffs between size, cost and efficiency. Inductor tolerance, core and copper loss will vary with the type of inductor selected and should be evaluated with a ML4771 under worst case conditions to determine its suitability. Several manufacturers supply standard inductance values in surface mount packages: Coilcraft Coiltronics Dale Sumida (847) 639-6400 (561) 241-7876 (605) 665-9301 (847) 956-0666
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ML4771
DESIGN CONSIDERATIONS
OUTPUT CAPACITOR The output capacitor filters the pulses of current from the switching regulator. Since the switching frequency will vary with inductance, the minimum output capacitance required to reduce the output ripple to an acceptable level will be a function of the inductor used. Therefore, to maintain an output voltage with less than 100mV of ripple at full load current, use the following equation: COUT = 44 L VOUT (2) (Continued) 0.5A and 1.5A. This fast change in current through the capacitor's ESL causes a high frequency (5ns) spike to appear on the output. After the ESL spike settles, the output still has a ripple component equal to the inductor discharge current times the ESR. To minimize these effects, choose an output capacitor with less than 10nH of ESL and 100mW of ESR. Suitable tantalum capacitors can be obtained from the following vendors: AVX Kemet Sprague (207) 282-5111 (846) 963-6300 (207) 324-4140
The output capacitor's Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL), also contribute to the ripple. Just after the NMOS transistor, Q1, turns off, the current in the output capacitor ramps quickly to between
1000
90
800 VOUT = 3.0V 80
VOUT = 3.0V EFFICIENCY (%)
IOUT (mA)
600
VOUT = 5.5V
400
VOUT = 5.5V
70
200 VIN = 2.4V 1 10 IOUT (mA) 100 1000
0 1.5
2.5
3.5 VIN (V)
4.5
5.5
60
Figure 4. IOUT vs. VIN Using the Circuit of Figure 8
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
160
VOUT = 5.5V 120
IIN (A)
80
40
VOUT = 3.0V
0 1.5
2.5
3.5 VIN (V)
4.5
5.5
Figure 6. No Load Input Current vs. VIN
Figure 7. Sample PC Board Layout
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ML4771
DESIGN CONSIDERATIONS
INPUT CAPACITOR Due to the high input current drawn at startup and possibly during operation, it is recommended to decouple the input with a capacitor with a value of 47F to 100F. This filtering prevents the input ripple from affecting the ML4771 control circuitry, and also improves the efficiency by reducing the I squared R losses during the charge cycle of the inductor. Again, a low ESR capacitor (such as tantalum) is recommended. It is also recommended that low source impedance batteries be used. Otherwise, the voltage drop across the source impedance during high input current situations will cause the ML4771 to fail to start-up or to operate unreliably. In general, for two cell applications the source impedance should be less than 200mW, which means that small alkaline cells should be avoided. SETTING THE OUTPUT VOLTAGE The adjustable output of the ML4771 requires an external feedback resistor divider to set VOUT. The output voltage can be determined from the following equation: VOUT (Continued)
DESIGN EXAMPLE
In order to design a boost converter using the ML4871, it is necessary to define a few parameters. For this example, assume that VIN = 3.0V to 3.6V, VOUT = 5.0V, and IOUT(MAX) = 500mA. First, it must be determined whether the ML4871 is capable of delivering the output current. This is done using Equation 1: IOUT( MAX) = 125 . . 30V 0.7A = 0.53A 50V .
Next, select an inductor. As previously mentioned, the recommended inductance is 10H. Make sure that the peak current rating of the inductor is at least 1.5A, and that the DC resistance of the inductor is in the range of 50 to 100mW. Then, the value of the output capacitor is determined using Equation 2: COUT = 44 10mH = 88mF 5.0V
1R + R 6 = 257 .
1 2
R2
(3)
The closest standard value would be a 100F capacitor with an ESR rating of 100mW. If such a low ESR value cannot be found, two 47F capacitors in parallel could also be used. Finally, the values of R1 and R2 are calculated using equation 3, assuming that R2 = 250kW: R1 =
where R1 and R2 are connected as shown in Figure 2. The value of R2 should be 250kW or less to minimize bias current errors. Choose an appropriate value for R2 and calculate R1.
LAYOUT
Good layout practices will ensure the proper operation of the ML4771. Some layout guidelines follow: * Use adequate ground and power traces or planes * Keep components as close as possible to the ML4771 * Use short trace lengths from the inductor to the VL1 and VL2 pins and from the output capacitor to the VOUT pin * Use a single point ground for the ML4771 ground pin, and the input and output capacitors
5.0 250kW - 250kW = 236kW 257 .
The complete circuit is shown in Figure 8. As mentioned previously, the use of an input supply bypass capacitor is highly recommended.
10H (Sumida CD75)
ML4771 VL1 PWR GND NC VL2 VOUT VOUT 100F 236k 250k
* Separate the ground for the converter circuitry from the ground of the load circuitry and connect at a single point A sample layout is shown in Figure 7.
VIN 100F
VIN GND SENSE
Figure 8. Typical Application Circuit
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ML4771
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) SEATING PLANE
0.004 - 0.010 (0.10 - 0.26)
0.015 - 0.035 (0.38 - 0.89)
0.006 - 0.010 (0.15 - 0.26)
ORDERING INFORMATION
PART NUMBER ML4771CS (End Of Life) ML4771ES (Obsolete) TEMPERATURE RANGE 0C to 70C -20C to 70C PACKAGE 8-Pin SOIC (S08) 8-Pin SOIC (S08)
(c) Micro Linear 2000. 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; 5,672,959; 5,689,167; 5,714,897; 5,717,798; 5,742,151; 5,747,977; 5,754,012; 5,757,174; 5,767,653; 5,777,514; 5,793,168; 5,798,635; 5,804,950; 5,808,455; 5,811,999; 5,818,207; 5,818,669; 5,825,165; 5,825,223; 5,838,723; 5.844,378; 5,844,941. Japan: 2,598,946; 2,619,299; 2,704,176; 2,821,714. Other patents are pending.
DS4771-01
2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 www.microlinear.com
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