Micro Linear ML4871 High current boost regulator Datasheet

July 2000
G
FEATURINperature Range
ML4871
Tem
ommercial
-20˚C to 70˚C
quipment
Handheld E
for Portable
Extended C
High Current Boost Regulator
GENERAL DESCRIPTION
FEATURES
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%.
■
Guaranteed full load start-up and operation
at 1.8V Input
■
Continuous conduction mode for high output current
■
Very low supply current (20µA 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
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.
BLOCK DIAGRAM
1
VL1
7
4
6
RESET
DETECT
VL2
+
COMP
–
VREF
VIN
2
SYNCHRONOUS
RECTIFIER
CONTROL
START-UP
VOUT
+
5
–
+
–
+
BOOST
CONTROL
–
PWR GND
1.25V
GND
8
3
1
ML4871
PIN CONFIGURATION
ML4871
8-Pin SOIC (S08)
VL1
1
8
PWR GND
VIN
2
7
RESET
GND
3
6
VL2
DETECT
4
5
VOUT
TOP VIEW
PIN DESCRIPTION
NO.
NAME
1
VL1
Boost inductor connection
2
VIN
3
4
2
FUNCTION
NO.
NAME
FUNCTION
5
VOUT
Boost regulator output
Battery input voltage
6
VL2
Boost inductor connection
GND
Ground
7
RESET
Output goes low when DETECT goes
below 1.25V
DETECT
Pulling this pin below 1.25V causes
the RESET pin to go low
8
PWR GND Return for the NMOS output transistor
ML4871
ABSOLUTE MAXIMUM RATINGS
OPERATING CONDITIONS
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.
Temperature Range
ML4871CS-X ............................................. 0ºC to 70ºC
ML4871ES-X .......................................... –20ºC to 70ºC
VIN Operating Range
ML4871CS-X ................................ 1.8V to VOUT – 0.2V
ML4871ES-X ................................. 2.0V to VOUT – 0.2V
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 .............................................. 150ºC
Storage Temperature Range...................... –65ºC to 150ºC
Lead Temperature (Soldering 10 sec) ....................... 260ºC
Thermal Resistance (qJA) .................................... 160ºC/W
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1).
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SUPPLY
IIN
IOUT(Q)
IL(Q)
VIN Current
VIN = VOUT – 0.2V
2
5
µA
VOUT Quiescent Current
DETECT = VIN
30
40
µA
DETECT = 0V
25
35
µA
1
µA
VL Quiescent Current
PFM REGULATOR
IL Peak Current
VOUT
1.2
1.4
1.7
A
-3 Suffix
3.30
3.35
3.40
V
-5 Suffix
4.95
5.05
5.15
V
See Figure 1, -3 Suffix
VIN = 2.4V, IOUT £ 400mA
3.20
3.25
3.40
V
See Figure 1, -5 Suffix
VIN = 2.4V, IOUT £ 220mA
4.85
4.95
5.15
V
DETECT Threshold
1.18
1.25
1.28
V
DETECT Hysteresis
25
35
45
mV
100
nA
Output Voltage
Load Regulation
IL(PEAK) = 0
RESET COMPARATOR
DETECT Bias Current
Note 1:
–100
RESET Output High Voltage
IRESET = –200µA
RESET Output Low Voltage
IRESET = 500µA
VOUT – 0.2
V
0.2
V
Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
3
ML4871
20µH
(Sumida CD75)
ML4871
VIN
VL1
PWR GND
VIN
RESET
GND
100µF
IOUT
VL2
DETECT
VOUT
200µF
Figure 1. Application Test Circuit.
IL
1
6
VL1
VIN
2
VL2
RSENSE
Q2
SYNCHRONOUS
RECTIFIER
CONTROL
START-UP
+
5
–
A3
+
+
BOOST
CONTROL
–
A1
Q1
–
ISET
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.
4
VOUT
VOUT
A2
1.25V
GND
3
ML4871
FUNCTIONAL DESCRIPTION
DESIGN CONSIDERATIONS
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.
OUTPUT CURRENT CAPABILITY
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 20µA
of supply current is drawn from the output. This allows the
part to remain efficient even when the load current drops
below 200µA.
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.
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 ML4871 is able to operate over a wide range of
inductor values. A value of 10µH is a good choice, but any
value between 5µH and 33µH 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 10µH
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 µH 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
(847) 639-6400
Coiltronics
(561) 241-7876
Dale
(605) 665-9301
Sumida
(847) 956-0666
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.
5
ML4871
DESIGN CONSIDERATIONS (Continued)
OUTPUT CAPACITOR
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.
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
Suitable tantalum capacitors can be obtained from the
following vendors:
(2)
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
AVX
(207) 282-5111
Kemet
(846) 963-6300
Sprague
(207) 324-4140
90
1000
800
VOUT = 3.3V
EFFICIENCY (%)
IOUT (mA)
VOUT = 3.3V
600
VOUT = 5V
400
80
70
VOUT = 5V
200
0
1.0
2.0
3.0
60
5.0
4.0
VIN = 2.4V
1
10
VIN (V)
Figure 4. IOUT vs. VIN Using the Circuit of Figure 8
1000
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
90
VOUT = 5V
IIN (µA)
60
VOUT = 3.3V
30
0
1.0
2.0
3.0
4.0
5.0
VIN (V)
Figure 6. No Load Input Current vs. VIN
6
100
IOUT (mA)
ML4871
DESIGN CONSIDERATIONS (Continued)
LAYOUT
INPUT CAPACITOR
Good layout practices will ensure the proper operation of
the ML4871. Some layout guidelines follow:
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 47µF to 100µF.
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.
• 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
. ™
1R
A
+ RB
RB
6
(3)
7
VIN
RESET
2
RA
DETECT
4
+
COMP
RB
VREF
–
FROM
START-UP
CIRCUITRY
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.
The complete circuit is shown in Figure 9. As mentioned
previously, the use of an input supply bypass capacitor is
highly recommended.
First, it must be determined whether the ML4871 is
capable of delivering the output current. This is done using
Equation 1:
IOUT( MAX) = 125
. ™
10µH
(Sumida CD75)
30. V ™ 0.7A = 0.53A
50. V ML4871
Next, select an inductor. As previously mentioned, the
recommended inductance is 10µH. 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.
VIN
VL1
PWR GND
VIN
RESET
GND
100µF
DETECT
Finally, the value of the output capacitor is determined
using Equation 2:
COUT =
44 ™ 10mH
= 88mF
5.0V
VL2
VOUT
VOUT
100µF
Figure 9. Typical Application Circuit
The closest standard value would be a 100µF capacitor
with an ESR rating of 100mW. If such a low ESR value
cannot be found, two 47µF capacitors in parallel could
also be used.
IOUT(MAX) (mA)
VIN (V)
VOUT = 3.3V
VOUT = 5.0V
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
386.2
451.9
521.5
585.9
651.0
716.5
782.0
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
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
Table 1. Typical IOUT and Efficiency vs. VIN
8
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
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)
0º - 8º
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
OUTPUT VOLTAGE
TEMPERATURE RANGE
PACKAGE
ML4871CS-3
ML4871CS-5
3.3V
5.0V
0ºC to 70ºC
0ºC to 70ºC
8-Pin SOIC (S08)
8-Pin SOIC (S08)
ML4871ES-3
ML4871ES-5
3.3V
5.0V
–20ºC to 70ºC
–20ºC to 70ºC
8-Pin SOIC (S08)
8-Pin SOIC (S08)
© 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.
DS4871-01
2092 Concourse Drive
San Jose, CA 95131
Tel: 408/433-5200
Fax: 408/432-0295
www.microlinear.com
9
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