DN367 - Tiny Versatile Buck Regulators Operate from 3.6V to 36V Input

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Tiny Versatile Buck Regulators Operate from 3.6V to 36V Input
Design Note 367
Hua (Walker) Bai
Introduction
Linear Technology offers two new buck regulators that
operate from a wide input voltage range (3.6V to 36V)
and take so little space that they easily solve many difficult power supply problems. The LT®1936 and LT1933
are perfect for applications with disparate power inputs
or wide range input power supplies such as automotive
batteries, 24V industrial supplies, 5V logic supplies
and various wall adapters. Both parts are monolithic
current mode PWM regulators which provide excellent
line and load regulation and dynamic response. They
operate at a 500kHz switching frequency, enabling the
use of small, low cost inductors and ceramic capacitors,
resulting in low, predictable output ripple.
Small Size and Versatility
The LT1936 regulator includes a 1.9A power switch in
a tiny, thermally enhanced 8-lead MSOP. The LT1933
regulator includes an internal 0.75A power switch in a
tiny 6-lead ThinSOT™ package, which occupies less than
0.15in2 board space. The LT1936 offers the option of
external compensation for design flexibility or internal
compensation for compact solution size. Both parts offer
soft-start via the SHDN pin, thus reducing maximum
inrush currents during start-up. Both parts also have
a very low, 2μA shutdown current which significantly
extends battery life in applications that spend long
periods of time in sleep or shutdown mode. During
D2
CMDSH-4E
1
VIN
4.5V
TO 36V C6
22μF
OPT
2
+
C1
2.2μF
50V
5
8
C5
0.22μF
BOOST
LT1936
VIN
SW
3
SHDN
COMP
VC
C1: TDK C3225X5R1H225M
C2: TDK C3225X5R0J476M
7
C6: SANYO 50CV22BS
D1: DIODES, INC DFLS140L
L1: TOKO D63CB P/N: A916CY-100M
FB
GND
4
L1
10μH
R2
17.4k 1%
6
D1
R1
10k
1%
Figure 1. Typical Application of LT1936 Accepts
4.5V to 36V and Produces 3.3V/1.2A
07/05/367_conv
VOUT
3.3V
1.2A
C2
47μF
6.3V
DN367 F01
short circuit, both parts offer frequency foldback, where
the switching frequency decreases by about a factor of
ten. The lower frequency allows the inductor current to
safely discharge, thereby preventing current runaway.
LT1936 Produces 3.3V at 1.2A from 4.5V to 36V
Figure 1 shows a typical application for the LT1936.
This circuit generates 3.3V at 1.2A from an input of
4.5V to 36V. With the same input voltage range, the
LT1933 circuit can supply 500mA. The typical output
voltage ripple of the Figure 1 circuit is less than 16mV
while efficiency is as high as 89%. Excellent transient
response is possible with either external compensation
or the internal compensation; this circuit uses internal
compensation to minimize component count. A high ESR
electrolytic capacitor, C6 in Figure 1, is recommended
to damp overshoot voltage in applications where the
circuit is plugged into a live input source through long
leads. For more information, refer to the LT1933 or
LT1936 data sheet.
Producing a Lower Output Voltage from the
LT1936
In order to fully saturate the internal NPN power
transistor of the LT1936, the BOOST pin voltage must
be at least 2.3V above the SW pin voltage. A charge
pump comprising D2 and C5 creates this headroom in
Figure 1. Nevertheless, when the output voltage is less
than 2.5V, different approaches are needed. Figure 2
shows one example. It allows VIN to go up to 36V and
generates 1.4A at 1.8V. In this circuit, Q2 serves as an
inexpensive Zener. The emitter-base breakdown voltage
of Q2 gives a stable 6V reference. The charging current
for the BOOST capacitor, C5, passes through the follower, Q1. R4, Q1 and Q2 limit the BOOST pin voltage
below its maximum rating of 43V. If the maximum VIN
in an application is less than 20V, simply tie VIN to D2
to allow a lower minimum input voltage.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
400
Q1 D2
C6
22μF
OPT
1
2
8
+
380
Q2
C1
2.2μF
50V
5
C5
0.22μF
BOOST
LT1936
3
SW
VIN
360
L1
10μH
R2
10k 1%
COMP
SHDN
6
FB
VC
GND
C1: TDK C3225X5R1H225M
4
7
C2, C8: TDK C3216X5R0J476M
C6: SANYO 50CV22BS
R3
D1: DIODES, INC DFLS140L
68.1k
D2: CENTRAL SEMI CMDSH-4E
L1: TOKO D63CB P/N: A916CY-100M
C3
Q1, Q2: DIODES, INC MMBT3904
150pF
VOUT
1.8V
1.4A
C2
47μF
6.3V
240
+
C4
10μF
50V
OPT
C1
2.2μF
50V
GND
BOOST
SHDN
SW
GND
FB
2
3
C2: TAIYO YUDEN JMK316BJ226KL-T
C3: TDK C3225X7R1H225KT
C4: SANYO 50CV10BJ
D1: ON SEMI MBRM140T3
L1: TOKO D63CB P/N: A916CY-100M
12
20
16
VIN (V)
24
28
DN367 F04
1
LT1933
4
8
4
DN367 F01
D2
VIN
260
C4
18pF
Negative Output from a Buck Regulator
The circuit shown in Figure 3 can generate a negative
voltage of –3.3V from a buck regulator such as the
LT1933. This circuit effectively sets the ground reference
of the LT1933 to –3.3V. The average inductor current
of this circuit is the summation of the input and output
current. The available output current is a function of
the input voltage as shown in Figure 4.
5
320
280
C8
47μF
6.3V
Figure 2. This Circuit Generates Lower Output Voltage
While Allowing Maximum Input Up to 36V
VIN
4.5V
TO 30V
340
300
R1
20k
1% C7 100pF
D1
IOUT (mA)
R4
10k
VIN
5V TO 25V
OPERATING
36V
TRANSIENT
6
C1
0.1μF
L1
10μH
D1 R2
16.5k 1%
C2
22μF
6.3V –VOUT
DN367 F03
3.3V
R3
10k
1%
Figure 3. This Circuit Produces –3.3V from 4.5V to 30V
Figure 4. Maximum Output Current of the Circuit
in Figure 3 as a Function of the Input Voltage
Tiny Circuit Generates 3.3V and 5V from a
Minimum 4.5V Supply
The circuit in Figure 5 is capable of generating two output
voltages from a minimum 4.5V supply. One output is
3.3V at 300mA, the other 5V at 50mA. The circuit is
especially useful in automotive cold crank conditions
when the battery voltage drops below 5V but both the
3.3V and 5V outputs need to be alive. If more current
is needed, the circuit can also be implemented using
the LT1936. Even though the input of the LT1761-5 is
unregulated, the 5V output is regulated by the LT1761-5
LDO. To maintain regulation, the 3.3V output current
should be always well above the 5V output current,
especially when VIN is low.
Conclusion
The LT1933 and LT1936 step-down switching regulators accept a wide variety of input sources as well as
offer compact, efficient and versatile solutions to many
otherwise hard-to-solve problems.
1
C5
4.7μF
3
VIN
OUT
LT1761-5
SHDN
BYP
5
4
C6
0.01μF
VOUT2
5V
C7 50mA
10μF
GND
5
VIN
4.5V TO 36V
VIN
BOOST
LT1933
+
GND
C4
10μF
50V
OPT
4
C3
2.2μF
50V
SHDN
SW
FB
GND
2
2
1
6
C1
0.1μF
L1
t
t
D1
VOUT1
3.3V
300mA
D3
D2
3
R3
10k
1%
R2
16.5k 1%
C2
22μF
6.3V
C2: TAIYO YUDEN JMK316BJ226KL-T
C3: TDK C3225X7R1H225KT
C4: SANYO 50CV10BJ
C5: TAIYO YUDEN TMK325BJ475MN
C7: TAIYO YUDEN JMK316BJ106ML
D1: DIODES, INC B160
D2: ON SEMI MBRM140T3
L1: COILTRONICS CTX25-1P
DN367 F05
Figure 5. From a Minimum 4.5V, This Circuit Produces Two Outputs at 3.3V/300mA and 5V/50mA
Data Sheet Download
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call (408) 432-1900
dn367f_conv LT/TP 0705 305K • PRINTED IN THE USA
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