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July 2000
ML4872
High Current Boost Regulator with Shutdown
GENERAL DESCRIPTION
The ML4872 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 ML4872
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 ML4872 requires only one
inductor and two capacitors to build a very small
regulator circuit capable of achieving conversion
efficiencies approaching 90%.
The SHDN input allows the user to stop the regulator from
switching and powers down the control circuitry.
FEATURES
s
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
s
s
s
s
s
s
s
BLOCK DIAGRAM
1
VIN
2
VL1
6
VL2
START-UP
SYNCHRONOUS
RECTIFIER
CONTROL
VOUT
+
–
5
+
–
BOOST
CONTROL
+
–
1.25V
SHDN
4
PWR GND
8
GND
3
1
ML4872
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.
V
OUT
.......................................................................... 7V
Voltage on any other pin ..... GND – 0.3V to V
OUT
+ 0.3V
Peak Switch Current (I
PEAK
) ......................................... 2A
Average Switch Current (I
AVG
) ..................................... 1A
Junction Temperature .............................................. 150ºC
Storage Temperature Range ...................... –65ºC to 150ºC
Lead Temperature (Soldering 10 sec) ...................... 260ºC
Thermal Resistance (q
JA
) .................................... 160ºC/W
OPERATING CONDITIONS
Temperature Range
ML4872CS-X .............................................. 0ºC to 70ºC
ML4872ES-X ........................................... –20ºC to 70ºC
V
IN
Operating Range
ML4872CS-X ................................ 1.8V to V
OUT
– 0.2V
ML4872ES-X ................................ 2.0V to V
OUT
– 0.2V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, V
IN
= Operating Voltage Range, T
A
= Operating Temperature Range (Note 1).
SYMBOL
SUPPLY
I
IN
I
OUT(Q)
V
IN
Current
V
OUT
Quiescent Current
V
IN
= V
OUT
– 0.2V
SHDN = 0V
SHDN = V
IN
I
L(Q)
V
L
Quiescent Current
2
25
15
5
35
22
1
µA
µA
µA
µA
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PFM REGULATOR
I
L
Peak Current
V
OUT
Output Voltage
I
L(PEAK)
= 0
-3 Suffix
-5 Suffix
Load Regulation
See Figure 1, -3 Suffix
V
IN
= 2.4V, I
OUT
= 400mA
See Figure 1, -5 Suffix
V
IN
= 2.4V, I
OUT
= 220mA
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
SHUTDOWN
Input Bias Current
Shutdown Threshold
V
SHDN
= high to low
–100
0.4
0.6
100
1.1
nA
V
Note 1:
Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
3
ML4872
FUNCTIONAL DESCRIPTION
The ML4872 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 ML4872 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 ML4872 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, I
SET
. 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 I
SET
current by switching Q1
on and off. The peak inductor current is limited by the
controller to about 1.5A.
At light loads, I
SET
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 V
L2
rises above V
OUT
, amplifier
A2 allows the PMOS transistor Q2 to turn on. In
discontinuous operation, (where I
L
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.
I
OUT( MAX)
=
125
.
V
V
IN( MIN)
OUT
0.7A
(1)
INDUCTOR SELECTION
The ML4872 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 trade-
offs 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 ML4872 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
SHUTDOWN
The SHDN pin should be held low for normal operation.
Raising the shutdown voltage above the threshold level
will disable the synchronous rectifier and force I
SET
to
zero. This prevents switching from occurring, and the
output voltage becomes V
IN
– V
DIODE
.
5
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