July 2000
ML4865
High Voltage High Current Boost Regulator
1
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%.
BLOCK DIAGRAM
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
VL2
8
VOUT
1
SENSE
6
+
SHUTDOWN
CONTROL
7
SHDN
VIN
3
2.42V
START-UP
2
GND
VL1
4
SYNCHRONOUS
RECTIFIER
CONTROL
BOOST
CONTROL
SHDN
+
+
5
PWR GND
FEEDBACK
CONTROL
ML4865
2
PIN CONFIGURATION
PIN DESCRIPTION
PIN
NAME
FUNCTION
1
SENSE
Programming pin for setting the
output to any value lower than the
normal fixed voltage
2
GND
Ground
3
V
IN
Battery input voltage
4
V
L1
Boost inductor connection
PIN
NAME
FUNCTION
5
PWR GND
Return for the internal power
transistors
6
V
L2
Boost inductor connection
7
SHDN
Pulling this pin to V
IN
through an
external resistor shuts down the
regulator, isolating the load from
the input.
8
V
OUT
Boost regulator output
ML4865
8-Pin SOIC (S08)
1
2
3
4
8
7
6
5
SENSE
GND
VIN
VL1
VOUT
SHDN
VL2
PWR GND
TOP VIEW
ML4865
3
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, V
IN
= Operating Voltage Range, T
A
= Operating Temperature Range (Note 1)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SUPPLY
I
IN
V
IN
Current
SHDN = 0 or V
IN
10
25
A
V
OUT
Quiescent Current
V
OUT
= V
OUT(MAX)
+ 5%
20
30
A
V
L
Quiescent Current
0V < V
L2
< V
OUT
1
1
A
PFM REGULATOR
I
L(PEAK)
I
L
Peak Current
V
IN
= 5V
0.8
1.2
1.6
A
V
OUT
Output Voltage
See Figure 1
V
IN
= 5V, SENSE = open, I
OUT
= 0
11.72
12.1
12.48
V
Load Regulation
See Figure 1
V
IN
= 2.4V, I
OUT
= 40mA
11.52
12.0
V
V
IN
= 5V, I
OUT
= 160mA
11.52
12.0
V
FEEDBACK
Threshold Voltage
2.38
2.42
2.48
V
Input Bias Current
150
150
nA
SHUTDOWN
Threshold Voltage
V
SHDN
= high to low
0.4
0.8
1.6
V
Input Bias Current
150
150
nA
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions.
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 .............................................. 150C
Storage Temperature Range ...................... 65C to 150C
Lead Temperature (Soldering, 10 sec) ..................... 150C
Thermal Resistance (
q
JA
) .................................... 160C/W
OPERATING CONDITIONS
Temperature Range
ML4865CS-2 .............................................. 0C to 70C
ML4865ES-2 ............................................ 20C to 70C
V
IN
Voltage Range
Without external rectifier ............................ 1.8V to 6V
With external rectifier ................................ 1.8V to 10V
ML4865
4
Figure 1. Application Test Circuit
Figure 2. PFM Regulator Detailed Block Diagram
Figure 3. Inductor Current and Voltage Waveforms
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
I
L
0
V
L
I
L(MAX)
I
SET
V
OUT
0
VL2
L1
Q2
A2
Q1
8
+
VOUT
VOUT
6
C1
R1
R2
+
2.42V
VIN
A3
+
BOOST
CONTROL
SHUTDOWN
CONTROL
7
SHDN
VL1
4
VIN
3
150m
+
A1
1
SENSE
Q3
ISET
FEEDBACK
CONTROL
ML4865
IOUT
100F
100F
VIN
27H
(Sumida CD75)
SENSE
GND
VIN
VL1
VOUT
SHDN
VL2
PWR GND
ML4865
5
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.
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 150m
W 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
25A of supply current is drawn from the output. This
allows the part to remain efficient even when the load
current drops below 250A.
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.
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.
ML4865
6
900
700
500
300
100
0
I OUT
(mA)
VIN (V)
0
4
8
10
2
6
WITH
EXTERNAL
SCHOTTKY
WITHOUT
EXTERNAL
SCHOTTKY
Figure 4. Output Current vs. Input Voltage
Figure 5. Efficiency vs. Output Current
100
90
80
70
60
50
EFFICIENCY (%)
IOUT (mA)
1
10
100
1000
VOUT = 12V
with Schottky
without Schottky
VIN = 2V
VIN = 5V
VIN = 10V
DESIGN CONSIDERATIONS
INPUT VOLTAGE RANGE
The input voltage range determines whether an external
Schottky diode is necessary or optional. If the input
voltage is 6V or lower, the ML4865 can be operated as a
stand alone boost regulator with a shutdown that fully
isolates the input from the output. Adding an optional
Schottky diode extends the input voltage range up to 10V,
and improves the efficiency and the output current
capability. However, the external diode now provides a
leakage path from the input to the output during
shutdown.
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 dependent on
the input voltage. The full load efficiency may be as low
as 65% when the ML4865 is used without a Schottky
diode and can exhibit an input voltage dependence when
an external diode is used. 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 empirical equation:
I
V
V
I
OUT MAX
IN
OUT
IN
(
)
@
h
(A)
(1)
Where, for applications using the internal synchronous
rectifier:
I
V
V
V
OUT MAX
IN
OUT
IN
(
)
.
.
.
@
+
0 05
0 4
0 65
1
6
2
7
I
V
IN
IN
=
+
0 05
0 4
.
.
h = 0 65
.
And for applications using an external Schottky:
I
V
V
V
V
OUT MAX
IN
OUT
IN
IN
(
)
.
.
.
.
@
+
+
0 07
0 4
0 025
0 65
1
6
2
7 1
6
2
7
I
V
IN
IN
=
+
0 07
0 4
.
.
h =
+
0 025
0 65
.
.
V
IN
The curves and the equations are based on the operating
circuit shown in Figure 7. It is recommended to verify the
current capability and efficiency for the components
selected.
Figure 6. No Load Input Current vs. Input Voltage for the
Circuit of Figure 7
300
250
200
150
100
50
0
I IN
(mA)
VIN (V)
0
4
6
10
8
2
WITH
EXTERNAL
SCHOTTKY
WITHOUT
EXTERNAL
SCHOTTKY
ML4865
7
INDUCTOR SELECTION
The ML4865 is able to operate over a wide range of
inductor values. A value of 22H or 33H is a good
choice, but any value between 15H and 50H 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 15H 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 10m
W of resistance for each H 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 ML4865 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
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
(due to capacitance) at full load current, use the
following equation:
DESIGN CONSIDERATIONS
(Continued)
ML4865
D1
MBR0520L
VOUT
C2
47F
R1
1M
C1
47F
VIN
22H
(Sumida CD75)
SENSE
GND
VIN
VL1
VOUT
SHDN
VL2
PWR GND
Figure 7. Typical Application Circuit.
C
L
V
OUT
OUT
=
10
(F)
(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 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 200m
W of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
AVX
TPS Series
(207) 282-5111
Sprague
593D Series
(207) 324-4140
Kemet
T495 Series
(864) 963-6300
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 22F to 68F.
This filtering prevents the input ripple from affecting the
ML4865 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 ML4865 to fail to start-up or to operate
unreliably. In general, for two cell applications the source
impedance should be less than 200m
W, which means that
small alkaline cells should be avoided.
ML4865
8
SHUTDOWN
The SHDN pin is a high impedance input and is noise
sensitive. Either drive the SHDN input from a low
impedance source or bypass the pin to GND with a 10nF
ceramic capacitor.
SENSE
The SENSE pin should be left open or bypassed to ground
for normal operation. The output can be set to voltages
lower than the preset value by adding a resistor divider.
The output voltage can be determined from the following
equation:
V
R
R2
R2
OUT
=
+
2 42
1
.
(V)
(3)
where R1 and R2 are connected as shown in Figure 2. The
value of R2 should be 1M
W or less to minimize bias
current errors. Choose an appropriate value of R2 and
calculate the value of R1.
R
R2
V
OUT
1
2 42
1
=
-
.
(
W)
(4)
EXTERNAL SCHOTTKY RECTIFIER
Due to excessive power dissipation, an external Schottky
rectifier is required when operating at input voltages
above 6V. Even for applications where the input voltage is
below 6V, the use of an external rectifier may be
necessary to achive efficiency or output current
requirements.
DESIGN CONSIDERATIONS
(Continued)
If an external Schottky is required, look for a device with
a voltage rating of 20V or greater. The average forward
current rating should be at least 500mA, and the forward
voltage should be 600mV or less. Suitable Schottky
rectifiers can be obtained from the following vendors:
Diodes, Inc
B120
(805) 446-4800
Int'l Rectifier 10BQ040
(310) 322-3331
Motorola
MBR0520L
(602) 897-5056
LAYOUT
Good layout practices will ensure the proper operation of
the ML4865. Some layout guidelines follow:
Use adequate ground and power traces or planes
Keep components as close as possible to the ML4865
Use short trace lengths from the inductor to the V
L1
and
V
L2
pins and from the output capacitor to the V
OUT
pin
Use a single point ground for the ML4865 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.
Figure 8. Sample ML4865 Layout
ML4865
9
DESIGN EXAMPLE
In order to design a boost converter using the ML4865,
it is necessary to define the values of a few parameters.
For this example, assume the following design
parameters:
V
IN
= 4.75 to 5.25V
V
OUT
= 12V
I
OUT(MAX)
= 150mA
Shutdown required
First, it must be determined whether the ML4865 is
capable of delivering the output current without an
external Schottky rectifier. This is done using Equation 1:
I
V
V
V
OUT MAX
IN
OUT
IN
(
)
.
.
.
@
+
0 05
0 4
0 65
1
6
2
7
I
mA
OUT MAX
(
)
.
.
.
.
.
@
+
=
5 25
12
0 05 5 25
0 4
0 65 188
0
5
2
7
Next, select an inductor. As previously mentioned, the
recommended inductance is 22H. 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
110 to 220m
W. A Sumida CD75-220 meets these
requirements.
Finally, the value of the output capacitor is determined
using Equation 2:
C
L
V
H
V
F
OUT
OUT
=
=
=
10
10 22
12
18 3
m
m
.
The closest standard value would be a 22F capacitor
with an ESR rating of 200m
W. An AVX
TPSD226M025R0200 would be a good choice.
As mentioned previously, the use of an input supply
bypass capacitor is highly recommended. Since the
output capacitance meets the minimum input
capacitance recommended it can also be used for the
input.
ML4865
1
DS4865-01
PHYSICAL DIMENSIONS
inches (millimeters)
2092 Concourse Drive
San Jose, CA 95131
Tel: 408/433-5200
Fax: 408/432-0295
www.microlinear.com
ORDERING INFORMATION
PART NUMBER
OUTPUT VOLTAGE
TEMPERATURE RANGE
PACKAGE
ML4865CS-2
12V
0C to 70C
8-Pin SOIC (S08)
ML4865ES-2 (Obsolete)
12V
20C to 70C
8-Pin SOIC (S08)
SEATING PLANE
0.148 - 0.158
(3.76 - 4.01)
PIN 1 ID
0.228 - 0.244
(5.79 - 6.20)
0.189 - 0.199
(4.80 - 5.06)
0.012 - 0.020
(0.30 - 0.51)
0.050 BSC
(1.27 BSC)
0.015 - 0.035
(0.38 - 0.89)
0.059 - 0.069
(1.49 - 1.75)
0.004 - 0.010
(0.10 - 0.26)
0.055 - 0.061
(1.40 - 1.55)
8
0.006 - 0.010
(0.15 - 0.26)
0 - 8
1
0.017 - 0.027
(0.43 - 0.69)
(4 PLACES)
Package: S08
8-Pin SOIC
Micro Linear 1998.
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; 5,714,897;
5,717,798; 5,742,151; 5,754,012; 5,757,174. 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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