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ML4865
High Voltage High Current Boost Regulator
Features
• Guaranteed full load start-up and operation at 1.8V input
• Continuous conduction mode for high output current
• Very low quiescent current
• Pulse frequency modulation and internal synchronous
rectification for high efficiency
• Maximum switching frequency > 200kHz
• Minimum external components
• Low ON resistance internal switching FETs
• Fixed 12V output can be adjusted to lower output voltages
Block Diagram
4
V
L1
V
IN
3
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%.
6
V
L2
SHUTDOWN
CONTROL
SHDN
7
START-UP
+
–
SHDN
BOOST
CONTROL
PWR GND
5
SYNCHRONOUS
RECTIFIER
CONTROL
+
–
2.42V
+
–
FEEDBACK
CONTROL
GND
2
V
OUT
8
SENSE
1
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ML4865 PRODUCT SPECIFICATION
Pin Configuration
ML4865
8-Pin SOIC (S08)
V
SENSE
GND
V
V
L1
IN
1
2
3
4
TOP VIEW
8
OUT
SHDN
7
V
6
L2
PWR GND
5
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.
3VINBattery input voltage.
4VL1Boost inductor connection.
5 PWR GND Return for the internal power transistors.
6V
L2
7 SHDN Pulling this pin to V
8V
OUT
Boost inductor connection.
through an external resistor shuts down the regulator,
IN
isolating the load from the input.
Boost regulator output.
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.
Parameter Min. Max. Units
Voltage on any other Pin GND – 0.3 16.5 V
Peak Switch Current (I
Average Switch Current (I
) 2 A
PEAK
)1A
AVG
Junction Temperature 150 °C
Storage Temperature Range -65 150 °C
Lead Temperature (soldering, 10s) 150 °C
Thermal Resistance (θJA) 160 °C/W
Operating Conditions
Parameter Min. Max. Units
Temperature Range ML4865CS-2 0 70 °C
VIN Voltage Range
Without external rectifier
With external rectifier
1.8
1.8
10
6
V
V
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PRODUCT SPECIFICATION ML4865
Electrical Characteristics
Unless otherwise specified, V
Symbol Parameter Conditions Min. Typ. Max. Units
Supply
I
IN
VIN Current SHDN = 0 or V
V
Quiescent Current V
OUT
VL Quiescent Current 0V < VL2 < V
PFM Regulator
I
L(PEAK)
V
OUT
IL Peak Current VIN = 5V 0.8 1.2 1.6 A
Output Voltage See Figure 1
Load Regulation See Figure 1
Feedback
Threshold Voltage 2.38 2.42 2.48 V
Input Bias Current -150 150 nA
Shutdown
Threshold Voltage V
Input Bias Current -150 150 nA
= Operating Voltage Range, TA = Operating Temperature Range (Note 1)
IN
IN
= V
OUT
OUT(MAX)
VIN = 5V, SENSE = Open, I
VIN = 2.4V, I
VIN = 5V, I
SHDN
OUT
= HIGH to LOW 0.4 0.8 1.6 V
+ 5% 20 30 µA
OUT
= 40mA
OUT
= 160mA
= 0 11.72 12.1 12.48 V
OUT
-1 1 µA
11.52
11.52
10 25 µA
12.0
12.0
V
V
Note:
1. Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions.
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ML4865 PRODUCT SPECIFICATION
V
IN
100µF
ML4865
SENSE
GND
V
V
(Sumida CD75)
IN
L1
V
SHDN
PWR GND
27µH
OUT
V
L2
I
OUT
100µF
Figure 1. Application Test Circuit
V
L1
A1
+
–
BOOST
CONTROL
L1
6
Q1
V
L2
Q3
Q2
A2
FEEDBACK
SHUTDOWN
CONTROL
+
–
CONTROL
SHDN
V
OUT
SENSE
7
C1
V
+
OUT
–
8
R1
1
V
IN
3
V
IN
150mΩ
4
+
–
I
SET
A3
2.42V
Figure 2. PFM Regulator Detailed Block Diagram
I
L
0
V
OUT
V
L
0
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
Figure 3. Inductor Current and Voltage Waveforms
I
L(MAX)
I
SET
R2
4 REV. 1.0.2 8/10/01
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PRODUCT SPECIFICATION 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.
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
sured through a 150mΩ resistor which is amplified by A1.
The boost control block matches the average inductor current
to a multiple of the I
SET
The peak inductor current is limited by the controller to
about 1.2A.
At light loads, I
will momentarily reach zero after an
SET
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.
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 V
PMOS transistor Q2 to turn on. In discontinuous operation,
(where I
always returns to zero), A2 uses the resistive drop
L
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 inductor current is mea-
SET
current by switching Q1 on and off.
, amplifier A2 allows the
OUT
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.
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:
V
I
OUT MAX
Where, for applications using the internal synchronous
rectifier:
I
OUT MAX
(
IV
=×+005 04..
IN IN
η=065.
And for applications using an external Schottky:
IN
V
OUT
V
IN
OUT
I
IN()
(A)
(
...≅××+×005 04 065
(
V
IN()
≅××η
V
(
(1)
(
(
Shutdown
The SHDN pin should be held low for normal operation.
I
OUT MAX
V
IN
(
... .≅× ×+× ×+007 04 0025 065
(
V
OUT
(
VV
IN IN()
(
(
(
Raising the shutdown voltage above the threshold level will
disable the synchronous rectifier, Q2 and Q3, and force I
SET
(
=×+007 04..
IV
IN IN
(
to zero. This prevents switching from occurring and
disconnects the body diode of Q2 from the output. As a
η= × +0 025 065..V
(
(
IN
result, the output voltage is allowed to drop below the input
voltage and current is prevented from flowing from the input
to the output.
REV. 1.0.2 8/10/01 5
((
Page 6
ML4865 PRODUCT SPECIFICATION
The curves and the equations are based on the operating
900
700
500
(mA)
OUT
I
300
100
0
04810
26
WITHOUT
EXTERNAL
SCHOTTKY
VIN (V)
WITH
EXTERNAL
SCHOTTKY
Figure 4. Output Current vs. Input Voltage
circuit shown in Figure 7. It is recommended to verify the
current capability and efficiency for the components
selected.
V
IN
47µF
ML4865
C1
SENSE
GND
V
IN
V
L1
V
OUT
SHDN
V
L2
PWR GND
22µH
(Sumida CD75)
Figure 7. Typical Application Circuit
R1
1MΩ
D1
MBR0520L
Inductor Selection
The ML4865 is able to operate over a wide range of inductor
values. A value of 22µH or 33µH is a good choice, but any
100
90
VIN = 10V
V
OUT
= 12V
value between 15µH and 50µH 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 15µH will reduce the component’s footprint, but the efficiency and maximum output
current may drop.
80
VIN = 5V
It is important to use an inductor that is rated to handle 1.5A
peak currents without saturating. Also look for an inductor
70
VIN = 2V
EFFICIENCY (%)
60
with Schottky
50
1 10 100 1000
I
OUT
without Schottky
(mA)
Figure 5. Efficiency vs. Output Current
with low winding resistance. A good rule of thumb is to
allow 5 to 10mΩ 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 ML4865 under worst case
conditions to determine its suitability.
Several manufacturers supply standard inductance values in
surface mount packages:
Coilcraft (847) 639-6400
300
250
Coiltronics (561) 241-7876
Dale (605) 665-9301
Sumida (847) 956-0666
200
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
150
(mA)
IN
I
100
WITH
EXTERNAL
SCHOTTKY
to reduce the output ripple to an acceptable level will be a
50
WITHOUT
EXTERNAL
SCHOTTKY
0
0461082
VIN (V)
Figure 6. No Load Input Current vs.
Input Voltage for the Circuit of Figure 7
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:
L
C (F)
OUT
×10
=
V
OUT
V
OUT
C2
47µF
(2)
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PRODUCT SPECIFICATION ML4865
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 200mΩ 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 22µF to 68µF. 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 200mΩ, which means that small alkaline cells
should be avoided.
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:
R1 R2
V
OUT
2.42
+
R2
1=× −
V (V)
=×
2.42
OUT
where R1 and R2 are connected as shown in Figure 2. The
value of R2 should be 1MΩ or less to minimize bias current
errors. Choose an appropriate value of R2 and calculate the
value of R1.
R2 (Ω)
R1
(3)
(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.
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 Fairchild Semiconductor.
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 VL1 and
V
pins and from the output capacitor to the V
L2
• 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.
OUT
pin
Figure 8. Sample ML4865 Layout
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ML4865 PRODUCT SPECIFICATION
Design Example
Next, select an inductor. As previously mentioned, the recommended inductance is 22µH. Make sure that the peak cur-
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:
VIN= 4.75 to 5.25V
rent rating of the inductor is at least 1.5A, and that the DC
resistance of the inductor is in the range of 110 to 220mΩ. A
Sumida CD75-220 meets these requirements.
Finally, the value of the output capacitor is determined using
Equation 2:
V
= 12V
OUT
I
OUT(MAX)
= 150mA
C
10 × L
== =
OUT
V
OUT
10 × 22µH
12V
18.3µF
Shutdown required
The closest standard value would be a 22µF capacitor with
First, it must be determined whether the ML4865 is capable
of delivering the output current without an external Schottky
an ESR rating of 200mΩ. An AVX TPSD226M025R0200
would be a good choice.
rectifier. This is done using Equation 1:
As mentioned previously, the use of an input supply bypass
V
IN
I
OUT MAX
ImA
OUT MAX()
V
.
525
12
(
...≅××+×005 04 065
(
OUT
(
. .25 . .≅× × +×=
0 05 5 0 4 0 65 188
(
(
V
IN()
(
(
(
capacitor is highly recommended. Since the output capaci-
tance meets the minimum input capacitance recommended it
can also be used for the input.
8 REV. 1.0.2 8/10/01
Page 9
PRODUCT SPECIFICATION ML4865
Mechanical Dimensions inches (millimeters)
Package: S08
8-Pin SOIC
0.189 - 0.199
(4.80 - 5.06)
8
0.017 - 0.027
(0.43 - 0.69)
(4 PLACES)
0.055 - 0.061
(1.40 - 1.55)
PIN 1 ID
1
0.050 BSC
(1.27 BSC)
0.012 - 0.020
(0.30 - 0.51)
SEATING PLANE
0.148 - 0.158
(3.76 - 4.01)
0.059 - 0.069
(1.49 - 1.75)
0.228 - 0.244
(5.79 - 6.20)
0.004 - 0.010
(0.10 - 0.26)
0º - 8º
0.015 - 0.035
(0.38 - 0.89)
0.006 - 0.010
(0.15 - 0.26)
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Page 10
ML4865 PRODUCT SPECIFICATION
Ordering Information
Part Number Output Voltage Temperature Range Package
ML4865CS-2 12V 0°C to 70°C 8 Pin SOIC (S08)
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FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO
ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME
ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN;
NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body,
or (b) support or sustain life, and (c) whose failure to
perform when properly used in accordance with
2. A critical component in any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
instructions for use provided in the labeling, can be
reasonably expected to result in a significant injury of the
user.
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