July 2000
PRELIMINARY
ML4870
*
High Current Boost Regulator with Load Disconnect
1
GENERAL DESCRIPTION
The ML4870 is a continuous conduction boost regulator
designed for DC to DC conversion in multiple cell battery
power 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 ML4870
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 addtion, low quiescent current and variable
frequency operation result in high efficiency even at light
loads. The ML4870 requires only a few external
components to build a very small regulator capable of
achieving conversion efficiencies approaching 85%.
The SHDN input allows the user to stop the regulator from
switching, and provides complete isolation of the load
from the battery.
*Some Packages Are Obsolete
BLOCK DIAGRAM
FEATURES
s
Guaranteed full load start-up and operation at
1.8V input
s
Continuous conduction mode for high output current
s
Pulse Frequency Modulation and internal synchronous
rectification for high efficiency
s
Isolates the load from the input during shutdown
s
Minimum external components
s
Low ON resistance internal switching FETs
s
Low supply current
s
5V and 3.3V output versions
VL2
5
VOUT
6
+
SHUTDOWN
CONTROL
4
SHDN
VIN
2
2.4V
START-UP
3
GND
VL1
1
SYNCHRONOUS
RECTIFIER
CONTROL
BOOST
CONTROL
SHDN
+
+
8
PWR GND
ML4870
2
PIN CONFIGURATION
PIN DESCRIPTION
PIN
NAME
FUNCTION
1
V
L1
Boost inductor connection
2
V
IN
Battery input voltage
3
GND
Ground
4
SHDN
Pulling this pin to V
IN
shuts down the
regulator, isolating the load from the
input
PIN
NAME
FUNCTION
5
V
OUT
Boost regulator output
6
V
L2
Boost inductor connection
7
NC
No connection
8
PWR GND
Return for the NMOS output transistor
ML4870
8-Pin SOIC (S08)
1
2
3
4
8
7
6
5
VL1
VIN
GND
SHDN
PWR GND
NC
VL2
VOUT
TOP VIEW
ML4870
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(Q)
V
IN
Quiescent Current
V
IN
= V
OUT
- 0.2V, SHDN = 0V
3
6
A
V
IN
= SHDN = 2.4V, V
OUT
= 0V
0.3
1
A
I
OUT(Q)
V
OUT
Quiescent Current
SHDN = 0V
25
35
A
V
IN
= SHDN = 2.4V
14
20
A
V
OUT
= V
OUT(NOM)
PFM REGULATOR
I
PEAK
I
L
Peak Current
1.1
1.3
1.6
A
V
OUT
Output Voltage
I
OUT
= 0
-3 Suffix
3.30
3.35
3.40
V
See Figure 1
-5 Suffix
4.95
5.05
5.15
V
Load Regulation
-3 Suffix, V
IN
= 2.4V, I
OUT
= 400mA
3.20
3.25
3.40
V
-5 Suffix, V
IN
= 2.4V, I
OUT
= 220mA
4.85
4.95
5.15
V
SHUTDOWN
V
IL
Input Low Voltage
0.5
V
V
IH
Input High Voltage
V
IN
- 0.5
V
Input Bias Current
-100
100
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.
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 .............................................. 150C
Storage Temperature Range ..................... 65C to 150C
Lead Temperature (Soldering, 10 sec) ..................... 260C
Thermal Resistance (
q
JA
) .................................... 160C/W
OPERATING CONDITIONS
Temperature Range
ML4870CS-X .............................................. 0C to 70C
ML4870ES-X ........................................... -20C to 70C
V
IN
Operating Range ....................... 1.8V to V
OUT
- 0.2V
ML4870
4
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
IL
VL2
IL(MAX)
ISET
VOUT
0
0
VL2
5
VOUT
6
+
SHUTDOWN
CONTROL
4
SHDN
VIN
IL
2
2.4V
START-UP
3
GND
VL1
1
SYNCHRONOUS
RECTIFIER
CONTROL
BOOST
CONTROL
SHDN
+
+
8
PWR GND
A1
A2
A3
Q1
Q2
Q3
VOUT
RSENSE
Figure 1. Application Test Circuit
Figure 3. Inductor Current and Voltage Waveforms
Figure 2. PFM Regulator Block Diagram
ML4870
IOUT
100F
100F
VIN
27H
(SUMIDA CD75)
VL1
VIN
GND
SHDN
PWR GND
NC
VL2
VOUT
ML4870
5
FUNCTIONAL DESCRIPTION
The ML4870 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 P-channel
MOSFET to reduce losses, eliminate an external
component, and provide the means for load disconnect.
Also included on-chip are an N-channel MOSFET main
switch and current sense resistor.
REGULATOR OPERATION
The ML4870 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 including the key
external components 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 current sense resistor (R
SENSE
) 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 1.3A.
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. When 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 V
L2
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 point 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 ML4870 output can be shut down by pulling the
SHDN pin high (to V
IN
). When SHDN is high, the
regulator stops switching, the control circuitry is powered
down, and the body diode of the PMOS synchronous
rectifier is disconnected from the output. By switching
Q1, Q2, and Q3 off, the load is isolated from the input.
This allows the output voltage to be independent of the
input while in shutdown.
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 conversion
efficiency. The maximum inductor current is limited by
the boost converter to about 1A. The conversion
efficiency is determined mainly by the internal switches
as well as the external components, but can be
estimated at about 80%. The maximum output current
can be estimated by using the typical performance
curves shown in Figures 4 and 5, or by calculation using
the following equations:
I
V
V
A
OUT V
IN MIN
(
)
(
)
.
.
5
0 972
5
0144
=
-
(1)
I
V
V
A
OUT
V
IN MIN
( .
)
(
)
.
.
.
3 3
0 81
3 3
0 144
=
-
(2)
Since the maximum output current is based on when the
inductor current goes into current limit, it is not
recommended to operate the ML4870 at the maximum
output current continuously. Applications that have high
transient load currents should be evaluated under worst
case conditions to determine suitability.
INDUCTOR SELECTION
The ML4870 is able to operate over a wide range of
inductor values. A value of 10H is a good choice, but
any value between 5H and 33H is acceptable. As the
inductor value changes, the control circuitry will
automatically adjust to keep the inductor current under
control. Choosing an inductance value of less than 10H
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 1H 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 ML4870 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
ML4870
6
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
at full load current, use the following equation:
C
L
V
OUT
OUT
=
44
(3)
The output capacitor's Equivalent Series Resistance (ESR)
and Equivalent Series Inductance (ESL), also contribute to
the ripple. Just after the Q1 turns off, the current in the
DEISGN CONSIDERATIONS
(Continued)
Figure 4. I
OUT
vs. V
IN
Using the Circuit of Figure 8
Figure 5. Efficiency vs. I
OUT
Using the Circuit of Figure 8
1000
800
600
400
200
0
I OUT
(mA)
VIN (V)
1.0
2.0
3.0
5.0
4.0
VOUT = 3.3V
VOUT = 5V
90
80
70
60
EFFICIENCY (%)
IOUT (mA)
1
10
100
1000
VOUT = 3.3V
VOUT = 5V
VIN = 2.4V
350
300
250
200
150
100
50
0
I IN
(nA)
VIN (V)
1.0
3.0
5.0
7.0
Figure 6. Input Leakage vs. V
IN
in Shutdown
Figure 7. No Load Input Current vs. V
IN
output capacitor ramps quickly to between 0.5A and
1.3A. 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 less than
100m
W of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
AVX
(207) 282-5111
Kemet
(846) 963-6300
Sprague
(207) 324-4140
80
60
40
20
0
I IN
(A)
VIN (V)
1.0
2.0
3.0
5.0
4.0
VOUT = 3.3V
VOUT = 5V
ML4870
7
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 47F to 100F.
This filtering prevents the input ripple from affecting the
ML4870 control circuitry, and also improves the
efficiency by reducing the I
2
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 ML4870 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.
SHUTDOWN
The input levels of the SHDN pin are CMOS compatible.
To guarantee proper operation, SHDN must be pulled to
within 0.5V of GND or V
IN
to prevent excessive power
dissipation and possible oscillations. A graph of input
leakage current while in shudown is shown in Figure 6.
LAYOUT
Good layout practices will ensure the proper operation of
the ML4870. Some layout guidelines follow:
Use adequate ground and power traces or planes
Keep components as close as possible to the ML4870
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 ML4870 PWR GND
pin and the input and output capacitors, and connect
the GND pin to PWR GND using a separate trace
Separate the ground for the converter circuitry from the
ground of the load circuitry and connect at a single
point
Figure 8. Design Example Schematic Diagram
DEISGN CONSIDERATIONS
(Continued)
DESIGN EXAMPLE
In order to design a boost converter using the ML4870,
it is necessary to define the values of a few parameters.
For this example, we have assumed that V
IN
= 3.0V to
3.6V, V
OUT
= 5.0V, and I
OUT(MAX)
= 400mA
First, it must be determined whether the ML4870 is
capable of delivering the output current. This is done
using Equation 1:
I
V
V
mA
OUT MAX
(
)
.
.
.
.
=
-
=
0 972
30
5 0
0 144
439
Next, select an inductor:
As previously mentioned, it is the recommended
inductance is 10H. 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 100m
W.
Finally, the value of the output capacitor is determined
using Equation 3:
C
H
V
F
OUT
=
=
44 10
5 0
88
m
m
.
The closest standard value would be a 100F capacitor
with an ESR rating of 100m
W. If such a low ESR value
cannot be found, two 47F capacitors in parallel could
also be used.
The complete circuit is shown in Figure 8. As mentioned
previously, the use of an input supply bypass capacitor is
strongly recommended.
ML4870
VOUT
100F
100F
VIN
10H
(SUMIDA CD75)
VL1
VIN
GND
SHDN
PWR GND
NC
VL2
VOUT
ML4870
8
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.
11/24/97 Printed in U.S.A.
PHYSICAL DIMENSIONS
inches (millimeters)
ORDERING INFORMATION
PART NUMBER
OUTPUT VOLTAGE
TEMPERATURE RANGE
PACKAGE
ML4870CS-3
3.3V
0C to 70C
8-Pin SOIC (S08)
ML4870CS-5
5V
0C to 70C
8-Pin SOIC (S08)
ML4870ES-3 (Obsolete)
3.3V
-20C to 70C
8-Pin SOIC (S08)
ML4870ES-5(Obsolete)
5V
-20C to 70C
8-Pin SOIC (S08)
2092 Concourse Drive
San Jose, CA 95131
Tel: (408) 433-5200
Fax: (408) 432-0295
www.microlinear.com
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
DS4870-01
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