Micro Linear ML4872ES-3 High current boost regulator with shutdown Datasheet

July 2000
G
FEATURINperature Range
Tem
ommercial
Extended C
70˚C
to
C
0˚
-2
ment
dheld Equip
an
H
e
bl
ta
or
for P
ML4872
High Current Boost Regulator with Shutdown
GENERAL DESCRIPTION
FEATURES
The ML4872 is a 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 ML4872
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
■
Very low supply current (20µA output referenced) for
micropower operation
■
Pulse frequency modulation and internal synchronous
rectification for high efficiency
■
Maximum switching frequency > 200kHz
■
Minimum external components
■
Low ON resistance internal switching FETs
■
5V and 3.3V output versions
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 ML4872 requires only one
inductor and two capacitors to build a very small
regulator circuit capable of achieving conversion
efficiencies approaching 90%.
The SHDN input allows the user to stop the regulator from
switching and powers down the control circuitry.
BLOCK DIAGRAM
1
6
VL1
VIN
VL2
2
SYNCHRONOUS
RECTIFIER
CONTROL
START-UP
VOUT
+
5
–
+
–
+
BOOST
CONTROL
–
1.25V
SHDN
4
PWR GND
GND
8
3
1
ML4872
PIN CONFIGURATION
ML4872
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
1
VL1
Boost inductor connection
2
VIN
3
4
2
FUNCTION
PIN
NAME
FUNCTION
5
VOUT
Boost regulator output
Battery input voltage
6
V L2
Boost inductor connection
GND
Ground
7
NC
No connection
SHDN
Pulling this pin to VIN causes the
regulator to stop switching, and
powers down the control circuitry
8
PWR GND
Return for the NMOS output transistor
ML4872
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
ML4872CS-X .............................................. 0ºC to 70ºC
ML4872ES-X ........................................... –20ºC to 70ºC
VIN Operating Range
ML4872CS-X ................................ 1.8V to VOUT – 0.2V
ML4872ES-X ................................ 2.0V 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 .............................................. 150ºC
Storage Temperature Range ...................... –65ºC to 150ºC
Lead Temperature (Soldering 10 sec) ...................... 260ºC
Thermal Resistance (qJA) .................................... 160ºC/W
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1).
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SUPPLY
IIN
IOUT(Q)
IL(Q)
VIN Current
VIN = VOUT – 0.2V
2
5
µA
VOUT Quiescent Current
SHDN = 0V
25
35
µA
SHDN = VIN
15
22
µA
1
µA
VL Quiescent Current
PFM REGULATOR
IL Peak Current
V OUT
Output Voltage
Load Regulation
1.2
1.4
1.7
A
-3 Suffix
3.30
3.35
3.40
V
-5 Suffix
4.95
5.05
5.15
V
See Figure 1, -3 Suffix
VIN = 2.4V, IOUT = 400mA
3.20
3.25
3.40
V
See Figure 1, -5 Suffix
VIN = 2.4V, IOUT = 220mA
4.85
4.95
5.15
V
100
nA
1.1
V
IL(PEAK) = 0
SHUTDOWN
Input Bias Current
Shutdown Threshold
–100
VSHDN = high to low
0.4
0.6
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
3
ML4872
20µH
(Sumida CD75)
ML4870
VL1
VIN
100µF
PWR GND
VIN
NC
GND
VL2
SHDN
IOUT
VOUT
200µF
Figure 1. Application Test Circuit
IL
1
6
VL1
VIN
2
VL2
RSENSE
Q2
SYNCHRONOUS
RECTIFIER
CONTROL
START-UP
+
5
–
A3
+
BOOST
CONTROL
–
A1
+
Q1
–
2.4V
ISET
SHDN
4
PWR GND
GND
8
Figure 2. PFM Regulator Block Diagram
IL(MAX)
IL
ISET
0
VOUT
VL2
0
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
Figure 3. Inductor Current and Voltage Waveforms
4
VOUT
VOUT
A2
3
ML4872
FUNCTIONAL DESCRIPTION
DESIGN CONSIDERATIONS
The ML4872 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 and eliminate an external component. Also
included on-chip are an NMOS switch and current sense
resistor, further reducing the number of external
components, which makes the ML4872 very easy to use.
OUTPUT CURRENT CAPABILITY
REGULATOR OPERATION
The ML4872 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 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 50mW resistor 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 1.5A.
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. While the circuit is not switching, only
20µA of supply current is drawn from the output. This
allows the part to remain efficient even when the load
current drops below 200µA.
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 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 is
independent of A2, and is determined by the boost control
circuitry.
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
approximately 1.25A and the full load efficiency may be
as low as 70%. 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 equation:
IOUT( MAX) = 125
. ™
V
V
IN( MIN)
OUT
™ 0.7A
(1)
INDUCTOR SELECTION
The ML4872 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 is changed 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 µH 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 ML4872 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
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 and force ISET to
zero. This prevents switching from occurring, and the
output voltage becomes VIN – VDIODE.
5
ML4872
DESIGN CONSIDERATIONS
(Continued)
OUTPUT CAPACITOR
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 100mW of ESR.
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:
COUT =
44 ™ L
VOUT
Suitable tantalum capacitors can be obtained from the
following vendors:
(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
AVX
(207) 282-5111
Kemet
(846) 963-6300
Sprague
(207) 324-4140
90
1000
800
VOUT = 3.3V
EFFICIENCY (%)
IOUT (mA)
VOUT = 3.3V
600
VOUT = 5V
400
80
70
VOUT = 5V
200
0
1.0
2.0
3.0
60
5.0
4.0
VIN = 2.4V
1
10
VIN (V)
Figure 4. IOUT vs. VIN Using the Circuit of Figure 8
1000
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
90
VOUT = 5V
IIN (µA)
60
VOUT = 3.3V
30
0
1.0
2.0
3.0
4.0
VIN (V)
Figure 6. No Load Input Current vs. VIN
6
100
IOUT (mA)
5.0
ML4872
DESIGN 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
ML4872 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 ML4872 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.
LAYOUT
Good layout practices will ensure the proper operation of
the ML4872. Some layout guidelines follow:
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4872
• Use short trace lengths from the inductor to the VL1 and
VL2 pins and from the output capacitor to the VOUT pin
DESIGN EXAMPLE
In order to design a boost converter using the ML4872, it
is necessary to define a few parameters. For this example,
assume that VIN = 3.0V to 3.6V, VOUT = 5.0V, and
IOUT(MAX) = 500mA.
First, it must be determined whether the ML4871 is
capable of delivering the output current. This is done
using Equation 1:
IOUT( MAX) = 125
. ™
30. V ™ 0.7A = 0.53A
50. V Next, select an inductor. As previously mentioned, 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.
Finally, the value of the output capacitor is determined
using Equation 2:
COUT =
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
highly recommended.
• Use a single point ground for the ML4872 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 7.
10µH
(Sumida CD75)
ML4872
VIN
100µF
VL1
PWR GND
VIN
NC
GND
VL2
SHDN
VOUT
VOUT
100µF
Figure 7. Sample PC Board Layout
Figure 8. Typical Application Circuit
7
ML4872
IOUT(MAX) (mA)
VIN (V)
VOUT = 3.3V
VOUT = 5.0V
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
386.2
451.9
521.5
585.9
651.0
716.5
782.0
286.2
332.1
379.1
430.0
479.0
525.4
571.8
618.5
665.0
711.7
758.7
805.3
851.9
899.0
946.1
992.7
IOUT (mA)
VIN = 2.4V, VOUT = 3.3V
1.0
2.0
5.0
10.0
20.0
50.0
100.0
200.0
586.0
VIN = 2.4V, VOUT = 5.0V
1.0
2.0
5.0
10.0
20.0
50.0
100.0
200.0
485.0
Table 1. Typical IOUT and Efficiency vs. VIN
8
EFFICIENCY PERCENTAGE
82.0
84.4
87.0
87.6
87.9
88.3
88.6
88.2
65.1
84.4
87.0
87.7
88.4
88.9
89.1
88.9
87.5
71.6
ML4872
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
© Micro Linear 1998.
PART NUMBER
OUTPUT VOLTAGE
TEMPERATURE RANGE
PACKAGE
ML4872CS-3
ML4872CS-5
3.3V
5.0V
0ºC to 70ºC
0ºC to 70ºC
8-Pin SOIC (S08)
8-Pin SOIC (S08)
ML4872ES-3
ML4872ES-5
3.3V
5.0V
–20ºC to 70ºC
–20ºC to 70ºC
8-Pin SOIC (S08)
8-Pin SOIC (S08)
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; 5,767,653. 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.
DS4872-01
2092 Concourse Drive
San Jose, CA 95131
Tel: (408) 433-5200
Fax: (408) 432-0295
http://www.microlinear.com
9
Similar pages