July 2000
ML4865
High Voltage High Current Boost Regulator
GENERAL DESCRIPTION
The ML4865 is a high voltage, 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 ML4865 is capable of start-up with
input voltages as low as 1.8V and generates a 12V output
with output voltage accuracy of ±4%.
Unlike most boost regulators, the ML4865 isolates the
load from the battery when the SHDN pin is high. 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 ML4865 requires only one inductor and two
capacitors to build a very small regulator circuit capable
of achieving conversion efficiencies approaching 90%.
FEATURES
s
Guaranteed full load start-up and operation at 1.8V
input
s
Continuous conduction mode for high output current
s
Very low quiescent current
s
Pulse frequency modulation and internal synchronous
rectification for high efficiency
s
Maximum switching frequency > 200kHz
s
Minimum external components
s
Low ON resistance internal switching FETs
s
Fixed 12V output can be adjusted to lower output
voltages
BLOCK DIAGRAM
4
VL1
6
VL2
SHUTDOWN
CONTROL
SHDN
7
VIN
3
START-UP
SYNCHRONOUS
RECTIFIER
CONTROL
VOUT
+
–
8
+
–
SHDN
BOOST
CONTROL
+
–
FEEDBACK
CONTROL
2.42V
SENSE
1
PWR GND
5
GND
2
1
ML4865
PIN CONFIGURATION
ML4865
8-Pin SOIC (S08)
SENSE
GND
VIN
VL1
1
2
3
4
8
7
6
5
VOUT
SHDN
VL2
PWR GND
TOP VIEW
PIN DESCRIPTION
PIN
NAME
FUNCTION
PIN
NAME
FUNCTION
1
SENSE
Programming pin for setting the
output to any value lower than the
normal fixed voltage
Ground
5
6
PWR GND
V
L2
SHDN
Return for the internal power
transistors
Boost inductor connection
Pulling this pin to V
IN
through an
external resistor shuts down the
regulator, isolating the load from
the input.
Boost regulator output
2
3
4
GND
V
IN
V
L1
7
Battery input voltage
Boost inductor connection
8
V
OUT
2
ML4865
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.
Voltage on any Pin ......................... GND – 0.3V to 16.5V
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) ..................... 150ºC
Thermal Resistance (q
JA
) .................................... 160ºC/W
OPERATING CONDITIONS
Temperature Range
ML4865CS-2 .............................................. 0ºC to 70ºC
ML4865ES-2 ............................................ –20ºC to 70ºC
V
IN
Voltage Range
Without external rectifier ............................ 1.8V to 6V
With external rectifier ................................ 1.8V to 10V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, V
IN
= Operating Voltage Range, T
A
= Operating Temperature Range (Note 1)
SYMBOL
SUPPLY
I
IN
V
IN
Current
V
OUT
Quiescent Current
V
L
Quiescent Current
SHDN = 0 or V
IN
V
OUT
= V
OUT(MAX)
+ 5%
0V < V
L2
< V
OUT
–1
10
20
25
30
1
µA
µA
µA
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PFM REGULATOR
I
L(PEAK)
V
OUT
I
L
Peak Current
Output Voltage
Load Regulation
V
IN
= 5V
See Figure 1
V
IN
= 5V, SENSE = open, I
OUT
= 0
See Figure 1
V
IN
= 2.4V, I
OUT
= 40mA
V
IN
= 5V, I
OUT
= 160mA
0.8
11.72
11.52
11.52
1.2
12.1
12.0
12.0
1.6
12.48
A
V
V
V
FEEDBACK
Threshold Voltage
Input Bias Current
2.38
–150
2.42
2.48
150
V
nA
SHUTDOWN
Threshold Voltage
Input Bias Current
Note 1:
Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions.
V
SHDN
= high to low
0.4
–150
0.8
1.6
150
V
nA
3
ML4865
ML4865
SENSE
VIN
GND
VIN
100µF
VL1
VOUT
SHDN
VL2
PWR GND
100µF
IOUT
27µH
(Sumida CD75)
Figure 1. Application Test Circuit
L1
VIN
3
VIN
4
VL1
6
VL2
Q3
150mΩ
SHUTDOWN
CONTROL
SHDN
7
A1
+
–
Q2
A2
VOUT
+
–
8
C1
R1
+
VOUT
–
BOOST
CONTROL
Q1
SENSE
FEEDBACK
CONTROL
+
–
1
R2
2.42V
ISET
A3
Figure 2. PFM Regulator Detailed Block Diagram
I
L(MAX)
I
L
I
SET
0
V
OUT
V
L
0
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
Figure 3. Inductor Current and Voltage Waveforms
4
ML4865
FUNCTIONAL DESCRIPTION
The ML4865 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, eliminate an external component, and
allows for load disconnect. Also included on-chip are an
NMOS switch and current sense resistor, further reducing
the number of external components, which makes the
ML4865 very easy to use.
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.
REGULATOR OPERATION
The ML4865 is a variable frequency, current mode
switching regulator. Its unique control scheme converts
efficiently over more than three decades of load current.
A detailed block diagram of the boost converter is shown
in Figure 2.
Error amplifier A3 converts deviations in the desired
output voltage to a small current, I
SET
. The inductor
current is measured through a 150mW 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 1.2A.
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
25µA of supply current is drawn from the output. This
allows the part to remain efficient even when the load
current drops below 250µA.
SHUTDOWN
The SHDN pin should be held low for normal operation.
Raising the shutdown voltage above the threshold level
will disable the synchronous rectifier, Q2 and Q3, and
force I
SET
to zero. This prevents switching from occurring
and disconnects the body diode of Q2 from the output. As
a result, the output voltage is allowed to drop below the
input voltage and current is prevented from flowing from
the input to the output.
FEEDBACK
The SENSE pin should be left open or bypassed to ground
for normal operation. The addition of the resistor divider
R1 and R2 causes the input of error amplifier A3 to reach
the threshold voltage before the internal resistors do. This
allows the ML4865 to provide output voltages lower than
the preset 12V if desired.
5
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