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Extended C
Tem ommercial -20C to 70C quipment Handheld E for Portable
G FEATURIN rature Range pe
July 2000
ML4871
High Current Boost Regulator
GENERAL DESCRIPTION
The ML4871 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 ML4871 is capable of start-up with input voltages as low as 1.8V and is available in 5V and 3.3V output versions with an output voltage accuracy of 3%. 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 ML4871 requires only one inductor and two capacitors to build a very small regulator circuit capable of achieving conversion efficiencies approaching 90%. The circuit also contains a RESET output which goes low when the DETECT input drops below 1.25V.
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 5V and 3.3V output versions
s s
s
s s s s
BLOCK DIAGRAM
1 VL1 DETECT
4 RESET
+ -
COMP
7
6 VL2
VIN 2
VREF
START-UP
SYNCHRONOUS RECTIFIER CONTROL
VOUT
+ -
5
+ -
BOOST CONTROL
+ -
1.25V
PWR GND 8
GND 3
1
ML4871
PIN CONFIGURATION
ML4871 8-Pin SOIC (S08)
VL1 VIN GND DETECT 1 2 3 4 TOP VIEW 8 7 6 5 PWR GND RESET VL2 VOUT
PIN DESCRIPTION
NO. NAME FUNCTION NO. NAME FUNCTION
1 2 3 4
VL1 VIN GND DETECT
Boost inductor connection Battery input voltage Ground Pulling this pin below 1.25V causes the RESET pin to go low
5 6 7 8
VOUT VL2 RESET
Boost regulator output Boost inductor connection Output goes low when DETECT goes below 1.25V
PWR GND Return for the NMOS output transistor
2
ML4871
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 ML4871CS-X ............................................. 0C to 70C ML4871ES-X .......................................... -20C to 70C VIN Operating Range ML4871CS-X ................................ 1.8V to VOUT - 0.2V ML4871ES-X ................................. 2.0V to VOUT - 0.2V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1).
SYMBOL SUPPLY
IIN IOUT(Q) VIN Current VOUT Quiescent Current VIN = VOUT - 0.2V DETECT = VIN DETECT = 0V IL(Q) VL Quiescent Current 2 30 25 5 40 35 1 A A A A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PFM REGULATOR
IL Peak Current VOUT Output Voltage IL(PEAK) = 0 -3 Suffix -5 Suffix Load Regulation See Figure 1, -3 Suffix VIN = 2.4V, IOUT 400mA See Figure 1, -5 Suffix VIN = 2.4V, IOUT 220mA RESET COMPARATOR DETECT Threshold DETECT Hysteresis DETECT Bias Current RESET Output High Voltage RESET Output Low Voltage
Note 1:
1.2 3.30 4.95 3.20 4.85
1.4 3.35 5.05 3.25 4.95
1.7 3.40 5.15 3.40 5.15
A V V V V
1.18 25 -100 IRESET = -200A IRESET = 500A VOUT - 0.2
1.25 35
1.28 45 100
V mV nA V
0.2
V
Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
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ML4871
20H (Sumida CD75)
ML4871 VIN VL1 VIN 100F GND DETECT PWR GND RESET VL2 VOUT 200F IOUT
Figure 1. Application Test Circuit.
IL 1 VL1 6 VL2
VIN 2
RSENSE
START-UP
SYNCHRONOUS RECTIFIER CONTROL
Q2 A2
+ -
VOUT 5
VOUT
+ -
A3 BOOST CONTROL
+
A1
Q1
-
1.25V
PWR GND 8
ISET
GND 3
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.
4
ML4871
FUNCTIONAL DESCRIPTION
The ML4871 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 ML4871 very easy to use.
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:
REGULATOR OPERATION
The ML4871 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. RESET COMPARATOR An additional comparator is provided to detect low VIN, low VOUT, or any other error condition that the user may want to sense. The inverting input of the comparator is connected to the 1.25V reference, and the non-inverting input is connected to the DETECT pin. The output of this comparator is connected to the RESET pin of the device and can swing from VOUT to ground.
IOUT( MAX) = 125 .
V V
IN( MIN) OUT
0.7A
(1)
INDUCTOR SELECTION
The ML4871 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 ML4871 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
5
ML4871
DESIGN CONSIDERATIONS (Continued)
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) current in the output capacitor ramps quickly to between 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
1000
90
800 VOUT = 3.3V IOUT (mA) 600 VOUT = 5V EFFICIENCY (%) 80 VOUT = 3.3V
400
70 VOUT = 5V
200 VIN = 2.4V 1 10 IOUT (mA) 100 1000
0 1.0
2.0
3.0 VIN (V)
4.0
5.0
60
Figure 4. IOUT vs. VIN Using the Circuit of Figure 8
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
90
VOUT = 5V 60 IIN (A)
VOUT = 3.3V 30
0 1.0
2.0
3.0 VIN (V)
4.0
5.0
Figure 6. No Load Input Current vs. VIN
6
ML4871
DESIGN CONSIDERATIONS (Continued)
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 ML4871 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 ML4871 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.
LAYOUT
Good layout practices will ensure the proper operation of the ML4871. Some layout guidelines follow: * Use adequate ground and power traces or planes * Keep components as close as possible to the ML4871 * 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 ML4871 ground pin, and the input and output capacitors * 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 8.
BATTERY MONITORING
The condition of the batteries can be monitored using the DETECT pin. For primary batteries, the comparator can be used to signal that the batteries will soon need to be replaced. For rechargeable batteries, the comparator can be used to signal the start of a charging cycle. For input voltages greater than the minimum operating voltage, the RESET pin can be set to go low at a specified battery voltage by connecting a resistor divider across the battery stack and to the DETECT pin of the ML4871 as shown in Figure 7. The low battery trip voltage is determined by first choosing a minimum battery voltage, VIN(MIN), and then calculating the values of RA and RB: VIN( MIN) = 125 .
VIN 2 RA DETECT 4 RB VREF
+ -
COMP
7 RESET
1R
A
+ RB
6
FROM START-UP CIRCUITRY
RB
(3)
Figure 7. Battery Monitoring Circuit
The values of RA and RB should be sufficiently large to minimize the power dissipation in the divider. Also, use care when selecting the low battery trip point. Too high a trip voltage can lead to memory effects in the battery, while too low a trip point can lead to reduced service life or polarity reversal. Refer to the manufacturer's data sheets for more information on selecting and designing battery systems.
Figure 8. Sample PC Board Layout
7
ML4871
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 .
VIN
The complete circuit is shown in Figure 9. As mentioned previously, the use of an input supply bypass capacitor is highly recommended.
10H (Sumida CD75)
ML4871 VL1 VIN 100F GND DETECT PWR GND RESET VL2 VOUT 100F VOUT
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. Finally, the value of the output capacitor is determined using Equation 2: COUT = 44 10mH = 88mF 5.0V
Figure 9. Typical Application Circuit
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.
IOUT(MAX) (mA) VIN (V)
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8
IOUT (mA) VIN = 2.4V, VOUT = 3.3V 1.0 2.0 5.0 10.0 20.0 50.0 100.0 200.0 586.0 VIN = 2.4V, VOUT = 5.0V 1.0 2.0 5.0 10.0 20.0 50.0 100.0 200.0 485.0
EFFICIENCY PERCENTAGE
82.0 84.4 87.0 87.6 87.9 88.3 88.6 88.2 65.1 84.4 87.0 87.7 88.4 88.9 89.1 88.9 87.5 71.6
VOUT = 3.3V
386.2 451.9 521.5 585.9 651.0 716.5 782.0
VOUT = 5.0V
286.2 332.1 379.1 430.0 479.0 525.4 571.8 618.5 665.0 711.7 758.7 805.3 851.9 899.0 946.1 992.7
Table 1. Typical IOUT and Efficiency vs. VIN
8
ML4871
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
ML4871CS-3 ML4871CS-5 ML4871ES-3 ML4871ES-5
OUTPUT VOLTAGE
3.3V 5.0V 3.3V 5.0V
TEMPERATURE RANGE
0C to 70C 0C to 70C -20C to 70C -20C to 70C
PACKAGE
8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08)
(c) Micro Linear 1997. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners.
DS4871-01 2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 www.microlinear.com
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. Japan: 2,598,946; 2,619,299; 2,704,176. 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.
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