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

advertisement
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 ThinSOTTM 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
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
R2
17.4k 1%
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
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
VOUT
3.3V
1.2A
C2
47µF
6.3V
DN367 F01
extends battery life in applications that spend long periods
of time in sleep or shutdown mode. During 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
www.BDTIC.com/Linear
400
Q1 D2
VIN
5V TO 25V
OPERATING
36V
TRANSIENT
C6
22µF
OPT
380
Q2
2
8
+
360
1
C1
2.2µF
50V
5
C5
0.22µF
BOOST
LT1936
3
SW
VIN
L1
10µH
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
R2
10k 1%
COMP
C2
47µF
6.3V
260
240
DN367 F01
+
C4
10µF
50V
OPT
C1
2.2µF
50V
BOOST
SHDN
GND
C2: TAIYO YUDEN JMK316BJ226KL-T
C3: TDK C3225X7R1H225KT
C4: SANYO 50CV10BJ
D1: ON SEMI MBRM140T3
L1: TOKO D63CB P/N: A916CY-100M
20
16
VIN (V)
24
28
DN367 F04
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.
1
LT1933
4
12
Figure 4. Maximum Output Current of the Circuit
in Figure 3 as a Function of the Input Voltage
D2
VIN
8
4
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
SW
GND
FB
2
3
6
C1
0.1µF
L1
10µH
D1 R2
16.5k 1%
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.
C2
22µF
6.3V –VOUT
DN367 F03
3.3V
R3
10k
1%
Figure 3. This Circuit Produces –3.3V from 4.5V to 30V
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
•
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
For applications help,
call (408) 432-1900, Ext. 2759
http://www.linear.com
Linear Technology Corporation
dn367f LT/TP 0705 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
www.BDTIC.com/Linear
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2005