Micro Linear ML4870ES-3 High current boost regulator with load disconnect Datasheet

July 2000
PRELIMINARY
ML4870*
High Current Boost Regulator with Load Disconnect
GENERAL DESCRIPTION
FEATURES
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%.
■
Guaranteed full load start-up and operation at
1.8V input
■
Continuous conduction mode for high output current
■
Pulse Frequency Modulation and internal synchronous
rectification for high efficiency
■
Isolates the load from the input during shutdown
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%.
■
Minimum external components
■
Low ON resistance internal switching FETs
■
Low supply current
■
5V and 3.3V output versions
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
1
6
VL1
VL2
SHUTDOWN
CONTROL
SHDN
4
VIN
2
SYNCHRONOUS
RECTIFIER
CONTROL
START-UP
VOUT
+
5
–
+
–
SHDN
+
BOOST
CONTROL
–
PWR GND
2.4V
GND
8
3
1
ML4870
PIN CONFIGURATION
ML4870
8-Pin SOIC (S08)
VL1
1
8
PWR GND
VIN
2
7
NC
GND
3
6
VL2
SHDN
4
5
VOUT
TOP VIEW
PIN DESCRIPTION
PIN
NAME
FUNCTION
PIN
NAME
FUNCTION
1
VL1
Boost inductor connection
5
VOUT
Boost regulator output
2
VIN
Battery input voltage
6
VL2
Boost inductor connection
3
GND
Ground
7
NC
No connection
4
SHDN
Pulling this pin to VIN shuts down the
regulator, isolating the load from the
input
8
PWR GND
Return for the NMOS output transistor
2
ML4870
ABSOLUTE MAXIMUM RATINGS
OPERATING CONDITIONS
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.
Temperature Range
ML4870CS-X .............................................. 0°C to 70°C
ML4870ES-X ........................................... -20°C to 70°C
VIN Operating Range ....................... 1.8V to VOUT - 0.2V
VOUT ............................................................................................... 7V
Voltage on any other pin ..... GND - 0.3V to VOUT + 0.3V
Peak Switch Current (IPEAK) ......................................... 2A
Average Switch Current (IAVG) ..................................... 1A
Junction Temperature .............................................. 150°C
Storage Temperature Range ..................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) ..................... 260°C
Thermal Resistance (qJA) .................................... 160°C/W
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VIN = VOUT - 0.2V, SHDN = 0V
3
6
µA
VIN = SHDN = 2.4V, VOUT = 0V
0.3
1
µA
SHDN = 0V
25
35
µA
VIN = SHDN = 2.4V
VOUT = VOUT(NOM)
14
20
µA
1.1
1.3
1.6
A
SUPPLY
IIN(Q)
IOUT(Q)
VIN Quiescent Current
VOUT Quiescent Current
PFM REGULATOR
IPEAK
IL Peak Current
VOUT
Output Voltage
Load Regulation
IOUT = 0
-3 Suffix
3.30
3.35
3.40
V
See Figure 1
-5 Suffix
4.95
5.05
5.15
V
-3 Suffix, VIN = 2.4V, IOUT = 400mA
3.20
3.25
3.40
V
-5 Suffix, VIN = 2.4V, IOUT = 220mA
4.85
4.95
5.15
V
0.5
V
SHUTDOWN
VIL
Input Low Voltage
V IH
Input High Voltage
VIN - 0.5
Input Bias Current
-100
V
100
nA
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
3
ML4870
27µH
(SUMIDA CD75)
ML4870
VIN
100µF
VL1
PWR GND
VIN
NC
GND
VL2
SHDN
IOUT
VOUT
100µF
Figure 1. Application Test Circuit
IL
1
6
VL1
VL2
SHUTDOWN
CONTROL
Q3
VIN
2
4
RSENSE
Q2
SYNCHRONOUS
RECTIFIER
CONTROL
START-UP
+
5
A3
+
BOOST
CONTROL
–
A1
SHDN
+
Q1
–
PWR GND
2.4V
GND
8
3
Figure 2. PFM Regulator Block Diagram
IL(MAX)
IL
VOUT
VOUT
A2
–
ISET
0
VOUT
VL2
0
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
Figure 3. Inductor Current and Voltage Waveforms
4
SHDN
ML4870
FUNCTIONAL DESCRIPTION
DESIGN CONSIDERATIONS
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.
OUTPUT CURRENT CAPABILITY
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, ISET. The inductor current is
measured through a current sense resistor (RSENSE) which
is amplified by A1. The boost control block matches the
average inductor current to a multiple of the ISET current
by switching Q1 on and off. The peak inductor current is
limited by the controller to about 1.3A.
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:
IOUT( 5V) = 0.972 ™
IOUT( 3.3V) = 0.81 ™
V - 0144
5V . A
IN(MIN)
V - 0.144A
3.3V IN( MIN)
(1)
(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
At light loads, ISET 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
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 VL2 to go high.
As the voltage on VL2 rises above VOUT, amplifier A2
allows the PMOS transistor Q2 to turn on. In
discontinuous operation, (where IL 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 VIN). 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.
The ML4870 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 changes, 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 1µH of
inductance.
The final selection of the inductor will be based on tradeoffs 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
5
ML4870
DEISGN CONSIDERATIONS (Continued)
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:
44 ™ L
VOUT
C OUT =
(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
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
100mW of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
AVX
(207) 282-5111
Kemet
(846) 963-6300
Sprague
(207) 324-4140
90
1000
VOUT = 3.3V
800
EFFICIENCY (%)
IOUT (mA)
VOUT = 5V
600
VOUT = 3.3V
400
VOUT = 5V
80
70
200
0
1.0
2.0
3.0
4.0
60
5.0
VIN = 2.4V
1
10
VIN (V)
1000
IOUT (mA)
Figure 4. IOUT vs. VIN Using the Circuit of Figure 8
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
80
350
300
VOUT = 5V
60
250
200
IIN (µA)
IIN (nA)
100
150
100
40
VOUT = 3.3V
20
50
0
1.0
3.0
5.0
7.0
VIN (V)
Figure 6. Input Leakage vs. VIN in Shutdown
6
0
1.0
2.0
3.0
4.0
5.0
VIN (V)
Figure 7. No Load Input Current vs. VIN
ML4870
DEISGN CONSIDERATIONS (Continued)
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 47µF to 100µF.
This filtering prevents the input ripple from affecting the
ML4870 control circuitry, and also improves the
efficiency by reducing the I2R 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 200mW, which means that
small alkaline cells should be avoided.
10µH
(SUMIDA CD75)
ML4870
VL1
VIN
100µF
PWR GND
VIN
NC
GND
VL2
SHDN
VOUT
VOUT
100µF
Figure 8. Design Example Schematic Diagram
SHUTDOWN
DESIGN EXAMPLE
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 VIN to prevent excessive power
dissipation and possible oscillations. A graph of input
leakage current while in shudown is shown in Figure 6.
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 VIN = 3.0V to
3.6V, VOUT = 5.0V, and IOUT(MAX) = 400mA
LAYOUT
Good layout practices will ensure the proper operation of
the ML4870. Some layout guidelines follow:
First, it must be determined whether the ML4870 is
capable of delivering the output current. This is done
using Equation 1:
IOUT( MAX) = 0.972 ™
30. V - 0.144 = 439mA
5.0V • Use adequate ground and power traces or planes
Next, select an inductor:
• Keep components as close as possible to the ML4870
As previously mentioned, it is the recommended
inductance is 10µH. 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 100mW.
• Use short trace lengths from the inductor to the VL1 and
VL2 pins and from the output capacitor to the VOUT 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
Finally, the value of the output capacitor is determined
using Equation 3:
C OUT =
44 ™ 10mH
= 88mF
5.0V
The closest standard value would be a 100µF capacitor
with an ESR rating of 100mW. If such a low ESR value
cannot be found, two 47µF 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.
7
ML4870
PHYSICAL DIMENSIONS
inches (millimeters)
Package: S08
8-Pin SOIC
0.189 - 0.199
(4.80 - 5.06)
8
PIN 1 ID
0.148 - 0.158 0.228 - 0.244
(3.76 - 4.01) (5.79 - 6.20)
1
0.017 - 0.027
(0.43 - 0.69)
(4 PLACES)
0.050 BSC
(1.27 BSC)
0.059 - 0.069
(1.49 - 1.75)
0º - 8º
0.055 - 0.061
(1.40 - 1.55)
0.012 - 0.020
(0.30 - 0.51)
0.004 - 0.010
(0.10 - 0.26)
0.015 - 0.035
(0.38 - 0.89)
0.006 - 0.010
(0.15 - 0.26)
SEATING PLANE
ORDERING INFORMATION
PART NUMBER
OUTPUT VOLTAGE
TEMPERATURE RANGE
PACKAGE
ML4870CS-3
ML4870CS-5
3.3V
5V
0°C to 70°C
0°C to 70°C
8-Pin SOIC (S08)
8-Pin SOIC (S08)
ML4870ES-3 (Obsolete)
ML4870ES-5(Obsolete)
3.3V
5V
-20°C to 70°C
-20°C to 70°C
8-Pin SOIC (S08)
8-Pin SOIC (S08)
DS4870-01
© 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.
8
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www.microlinear.com
11/24/97 Printed in U.S.A.
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