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FEATURING
Extended Commercial Temperature Range -20C to 70C for Portable Handheld Equipment
ML4851*
Low Current, Voltage Boost Regulator
GENERAL DESCRIPTION
The ML4851 is a low power boost regulator designed for DC to DC conversion in 1 to 3 cell battery powered systems. The maximum switching frequency can exceed 100kHz, allowing the use of small, low cost inductors. The combination of BiCMOS process technology, internal synchronous rectification, variable frequency operation, and low supply current make the ML4851 ideal for 1 cell applications. The ML4851 is capable of start-up with input voltages as low as 1V and is available in 5V and 3.3V output versions with 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 ML4851 requires only one inductor and two capacitors to build a very small regulator circuit capable of achieving conversion efficiencies in excess of 90%. The circuit also contains a RESET output which goes low when the IC can no longer function due to low input voltage, or when the DETECT input drops below 200mV.
FEATURES
n Guaranteed full load start-up and operation at 1V input n Maximum switching frequency > 100kHz n Pulse Frequency Modulation (PFM) and internal
synchronous rectification for high efficiency
n Minimum external components n Low ON resistance internal switching FETs n Micropower operation n 5V and 3.3V output versions
*Some Packages Are Obsolete
BLOCK DIAGRAM
7 DETECT 4 VREF VIN
+ -
COMP
6 VL
RESET
1
START-UP VOUT
+ -
5
S R
Q Q
5s ONE SHOT
PWR GND 8
- +
VREF
REFERENCE
VREF
GND 3
VREF
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July, 2000
DATASHEET
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ML4851
PIN CONFIGURATION
ML4851 8-Pin SOIC (S08)
VIN VREF GND DETECT 1 2 3 4 8 7 6 5 PWR GND RESET VL VOUT
TOP VIEW
PIN DESCRIPTION
PIN NAME FUNCTION PIN NAME FUNCTION
1 2 3 4
VIN V REF GND DETECT
Battery input voltage 200mV reference output Analog signal ground Pulling this pin below VREF causes the RESET pin to go low
5 6 7
VOUT VL RESET
Boost regulator output Boost inductor connection Output goes low when regulation cannot be achieved, or when DETECT goes below VREF Return for the NMOS output transistor
8
PWR GND
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DATASHEET
ML4851
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, I(PEAK) ................................................... 1A Average Switch Current, I(AVG) ..................................... 250mA Junction Temperature .............................................. 150C Storage Temperature Range ...................... -65C to 150C Lead Temperature (Soldering 10 sec.) ..................... 260C
OPERATING CONDITIONS
Temperature Range ML4851CS-X .............................................. 0C to 70C ML4851ES-X ........................................... -20C to 70C VIN Operating Range ML4851CS-X ................................ 1.0V to VOUT - 0.2V ML4851ES-X ................................ 1.1V to VOUT - 0.2V Thermal Resistance (qJA) .................................... 160C/W
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1)
SYMBOL SUPPLY IIN IOUT(Q) IL REFERENCE VREF Output Voltage 0 < IREF < -5A 190 200 210 mV VIN Current VOUT Quiescent Current VL Quiescent Current VIN = VOUT - 0.2V 50 8 60 10 1 A A A PARAMETER CONDITIONS MIN TYP MAX UNITS
PFM REGULATOR tON Pulse Width 4.5 5 5.5 s
OUTPUT VOLTAGE V OUT Output Voltage tON = 0 at VOUT(MAX), 4.5s tON 5.5s at VOUT(MIN) Load Regulation See Figure 1, -3 Suffix VIN = 1.2V, IOUT 10mA See Figure 1, -3 Suffix VIN = 2.4V, IOUT 65mA See Figure 1, -5 Suffix VIN = 1.2V, IOUT 18mA See Figure 1, -5 Suffix VIN = 2.4V, IOUT 85mA Undervoltage Lockout Threshold RESET COMPARATOR DETECT Threshold DETECT Bias Current RESET Output High Voltage RESET Output Low Voltage IOH = 100A IOL = -100A 194 -100 VOUT - 0.2 0.2 200 206 100 mV nA V V -5 Suffix 4.85 3.2 3.2 4.85 4.85 5.0 3.3 3.3 5.0 5.0 0.85 5.15 3.4 3.4 5.15 5.15 0.95 V V V V V V -3 Suffix 3.2 3.3 3.4 V
Note 1: Limits are guaranteed by 100% testing, sampling or correlation with worst case test conditions.
DATASHEET
July 2000
3
ML4851
27H (Sumida CD75) VIN 100F
R
A
ML4851
VIN PWR GND RESET VL VOUT 100F IOUT VOUT VREF GND DETECT
1F
R
B
Figure 1. Application Test Circuit
L1 VIN 6 VL
START-UP
Q2
+ A2 -
VOUT
5
+
Q1 S R Q Q
-
C1
VOUT -
5s ONE SHOT
R1 A1
+
VREF
R2
Figure 2. PFM Regulator Block Diagram
INDUCTOR CURRENT
Q(ONE SHOT)
Q1 ON Q1 & Q2 OFF
Q2 ON
Q1 ON
Q2 ON
Figure 3. PFM Inductor Current Waveforms and Timing
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July, 2000
DATASHEET
ML4851
FUNCTIONAL DESCRIPTION
The ML4851 combines Pulse Frequency Modulation (PFM) and synchronous rectification to create a boost converter that is both highly efficient and simple to use. A PFM regulator charges a single inductor for a fixed period of time and then completely discharges before another cycle begins, simplifying the design by eliminating the need for conventional current limiting circuitry. Synchronous rectification is accomplished by replacing an external Schottky diode with an on-chip PMOS device, reducing switching losses and external component count. REGULATOR OPERATION A block diagram of the boost converter is shown in Figure 2. The circuit remains idle when VOUT is at or above the desired output voltage, drawing 50A from VIN, and 8A from VOUT through the feedback resistors R1 and R2. When VOUT drops below the desired output level, the output of amplifier A1 goes high, signaling the regulator to deliver charge to the output. Since the output of amplifier A2 is normally high, the flip-flop captures the A1 set signal and creates a pulse at the gate of the NMOS transistor Q1. The NMOS transistor will charge the inductor L1 for 5s, resulting in a peak current given by:
DESIGN CONSIDERATIONS
INDUCTOR Selecting the proper inductor for a specific application usually involves a trade-off between efficiency and maximum output current. Choosing too high a value will keep the regulator from delivering the required output current under worst case conditions. Choosing too low a value causes efficiency to suffer. It is necessary to know the maximum required output current and the input voltage range to select the proper inductor value. The maximum inductor value can be estimated using the following formula:
LMAX =
VIN( MIN) 2 x t ON( MIN) x 2 x VOUT x IOUT( MAX )
(2)
where h is the efficiency, typically between 0.8 and 0.9. Note that this is the value of inductance that just barely delivers the required output current under worst case conditions. A lower value may be required to cover inductor tolerance, the effect of lower peak inductor currents caused by resistive losses, and minimum dead time between pulses. Another method of determining the appropriate inductor value is to make an estimate based on the typical performance curves given in Figures 4 and 5. Figure 4 shows maximum output current as a function of input voltage for several inductor values. These are typical performance curves and leave no margin for inductance and ON-time variations. To accommodate worst case conditions, it is necessary to derate these curves by at least 10% in addition to inductor tolerance. For example, a two cell to 5V application requires 60mA of output current while using an inductor with 15% tolerance. The output current should be derated by 25% to 80mA to cover the combined inductor and ON-time tolerances. Assuming that 2V is the end of life voltage of a two cell input, Figure 4 shows that with a 2V input, the ML4851-5 delivers 80mA with an 18H inductor. Figure 5 shows efficiency under the conditions used to create Figure 4. It can be seen that efficiency is mostly independent of input voltage and is closely related to inductor value. This illustrates the need to keep the inductor value as high as possible to attain peak system efficiency. As the inductor value goes down to 10H, the efficiency drops to around 75%. With 33H, the efficiency exceeds 90% and there is little room for improvement. At values greater than 33H, the operation of the synchronous rectifier becomes unreliable at low input voltages because the inductor current is so small that it is difficult for the control circuitry to detect. The data used to generate Figures 4 and 5 is provided in Table 1.
IL(PEAK )
t x VIN 5s x VIN = ON = L1 L1
(1)
For reliable operation, L1 should be chosen so that IL(PEAK) does not exceed 1A. When the one-shot times out, the NMOS transistor releases the VL pin, allowing the inductor to fly-back and momentarily charge the output through the body diode of PMOS transistor Q2. But, as the voltage across the PMOS transistor changes polarity, its gate will be driven low by the current sense amplifier A2, causing Q2 to short out its body diode. The inductor then discharges into the load through Q2. The output of A2 also serves to reset the flipflop and one-shot in preparation for the next charging cycle. A2 releases the gate of Q2 when its current falls to zero. If VOUT is still low, the flip-flop will immediately initiate another pulse. The output capacitor (C1) filters the inductor current, limiting output voltage ripple. Inductor current and one-shot waveforms are shown in Figure 3. RESET COMPARATOR An additional comparator is provided to detect low VIN, or any other error condition that is important to the user. The inverting input of the comparator is internally connected to VREF, while the non-inverting input is provided externally at the DETECT pin. The output of the comparator is the RESET pin, which swings from VOUT to GND when an error is detected. (Refer to Block Diagram)
DATASHEET
July 2000
5
ML4851
250
ML4851-3
L = 18H
250 ML4851-5 200
L = 18H
L = 33H
200
L = 10H IOUT MAX (mA) L = 33H
IOUT MAX (mA)
150 L = 56H 100
150 L = 10H 100
L = 56H
50
50
0 1.0
1.5
2.0 VIN (V)
2.5
3.0
0 1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
VIN (V)
Figure 4. Output Current vs. Input Voltage
95
95
ML4851-3 L = 56H
ML4851-3
L = 56H
EFFICIENCY at IOUT MAX (%)
EFFICIENCY at IOUT MAX (%)
90 L = 33H 85 L = 18H 80 L = 10H 75
90 L = 33H 85 L = 18H 80 L = 10H 75
70 1.0
1.5
2.0 VIN (V)
2.5
3.0
70 1.0
1.5
2.0 VIN (V)
2.5
3.0
Figure 5. Typical Efficiency as a Function of VIN
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July, 2000
DATASHEET
ML4851
DESIGN CONSIDERATIONS
(Continued) can be over 1V in magnitude. After the ESL spike settles, the output voltage still has a ripple component equal to the inductor discharge current times the ESR. This component will have a sawtooth shape and a peak value equal to the peak inductor current times the ESR. ESR also has a negative effect on efficiency by contributing I2R losses during the discharge cycle. An output capacitor with a capacitance of 100F, an ESR of less than 0.1W, and an ESL of less than 5nH is a good general purpose choice. Tantalum capacitors which meet these requirements can be obtained from the following suppliers: Matsuo Sprague (207) 282-5111 (207) 324-4140 After the appropriate inductor value is chosen, it is necessary to find the minimum inductor current rating required. Peak inductor current is determined from the following formula:
IL(PEAK ) =
t ON( MAX ) x VIN( MAX ) LMIN
(3)
In the two cell application previously described, a maximum input voltage of 3V would give a peak current of 1A. When comparing various inductors, it is important to keep in mind that suppliers use different criteria to determine their ratings. Many use a conservative current level, where inductance has dropped to 90% of its normal level. In any case, it is a good idea to try inductors of various current ratings with the ML4851 to determine which inductor is the best choice. Check efficiency and maximum output current, and if a current probe is available, look at the inductor current to see if it looks like the waveform shown in Figure 3. For additional information, see Applications Note 29, "Choosing an Inductor for Your ML4861 Application." Suitable inductors can be purchased from the following suppliers: Coilcraft Coiltronics Dale Sumida (847) 639-6400 (561) 241-7876 (605) 665-9301 (847) 956-0666
If ESL spikes are causing output noise problems, an EMI filter can be added in series with the output. INPUT CAPACITOR Unless the input source is a very low impedance battery, it will be necessary to decouple the input with a capacitor with a value of between 47F and 100F. This provides the benefits of preventing input ripple from affecting the ML4851 control circuitry, and it also improves efficiency by reducing I2R losses during the charge and discharge cycles of the inductor. Again, a low ESR capacitor (such as tantalum) is recommended. REFERENCE CAPACITOR Under some circumstances input ripple cannot be reduced effectively. This occurs primarily in applications where inductor currents are high, causing excess output ripple due to "pulse grouping", where the charge-discharge pulses are not evenly spaced in time. In such cases it may be necessary to decouple the reference pin (VREF) with a small 10nF to 100nF ceramic capacitor. This is particularly true if the ripple voltage at VIN is greater than 100mV. SETTING THE RESET THRESHOLD To use the RESET comparator as an input voltage monitor, as shown in Figure 1, it is necessary to use an external resistor divider tied to the DETECT pin as shown in the block diagram. The resistor values RA and RB can be calculated using the following equation:
XFMRS, Inc. (317) 834-1066 OUTPUT CAPACITOR The choice of output capacitor is also important, as it controls the output ripple and optimizes the efficiency of the circuit. Output ripple is influenced by three capacitor parameters: capacitance, ESR, and ESL. The contribution due to capacitance can be determined by looking at the change in capacitor voltage required to store the energy delivered by the inductor in a single charge-discharge cycle, as determined by the following formula:
VOUT
t ON2 x VIN2 = 2 x L x C x VOUT - VIN
1
6
(4)
For a 2.4V input, and 5V output, a 18H inductor, and a 47F capacitor, the expected output ripple due to capacitor value is 33mV. Capacitor Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL), also contribute to the output ripple due to the inductor discharge current waveform. Just after the NMOS transistor turns off, the output current ramps quickly to match the peak inductor current. This fast change in current through the output capacitor's ESL causes a high frequency (5ns) spike that
VIN( MIN) = 0.2 x
1R + R 6
A B
RB
(5)
The value of RB should be 100kW or less to minimize bias current errors. RA is then found by rearranging the equation:
DATASHEET
July 2000
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ML4851
DESIGN CONSIDERATIONS
R A = RB x
LAYOUT Good PC board layout practices will ensure the proper operation of the ML4851. Important layout considerations include:
V 0.2
IN( MIN)
-1

(Continued) (6)
TOP LAYER BOTTOM LAYER
n Use adequate ground and power traces or planes n Keep components as close as possible to the ML4851 n Use short trace lengths from the inductor to the VL pin
and from the output capacitor to the VOUT pin
n Use a single point ground for the ML4851 PWR GND
pin and the input and output capacitors, and connect GND to PWR GND with a separate trace
A sample PC board layout is shown in Figure 6. Figure 6. Sample PC Board Layout ML4851-3
VIN L = 10H 1.0 1.5 2.0 L = 18H 1.0 1.5 2.0 2.5 3.0 L = 33H 1.0 1.5 2.0 2.5 3.0 L = 56H 1.0 1.5 2.0 2.5 3.0 IOUT (mA) 45.8 108.3 184.1 30.1 70.9 125.5 185.7 243.4 17.6 42.7 76.1 120.4 159.6 10.6 25.9 47.6 75.8 108.0 EFFICIENCY PERCENTAGE 77.6 77.7 77.9 82.5 83.5 83.9 84.5 85.4 86.0 87.8 88.7 89.7 90.7 85.2 89.1 90.8 92.0 93.1
ML4851-5
V IN L = 10H 1.0 1.5 2.0 L = 18H 1.0 1.5 2.0 2.5 3.0 3.5 L = 33H 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 L = 56H 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 IOUT (mA) 24.2 68.0 123.1 15.7 43.3 80.4 125.3 169.9 236.9 9.1 24.8 47.4 74.5 106.9 147.5 190.0 227.8 5.5 13.9 28.5 45.7 67.1 92.5 122.1 149.6 EFFICIENCY PERCENTAGE 78.3 79.9 80.3 82.3 84.8 85.7 86.2 86.5 87.0 83.5 87.0 88.6 89.5 90.3 90.8 91.4 92.1 80.1 85.9 88.9 90.3 91.4 92.3 92.6 93.8
Table 1. Maximum Output Current and Efficiency vs. VIN
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July, 2000
DATASHEET
ML4851
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 ML4851CS-3 ML4851CS-5 ML4851ES-3 ML4851ES-5 ML4851IS-3 (obsolete) ML4851IS-5 (obsolete) OUTPUT VOLTAGE 3.3V 5.0V 3.3V 5.0V 3.3V 5.0V TEMPERATURE RANGE 0C to 70C 0C to 70C -20C to 70C -20C to 70C -40C to 85C -40C to 85C PACKAGE 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08)
Micro Linear Corporation 2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 www.microlinear.com
(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; 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.
DS4851-03
DATASHEET
July 2000
9

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