DN543 - Robust 140V VIN, 400mA Step-Down Regulator for Industrial, Telecom and Automotive Environments

Robust 140V VIN, 400mA Step-Down Regulator for Industrial,
Telecom and Automotive Environments
Design Note 543
Mike Shriver
The LTC ®7138 is a 400mA step-down regulator that
can operate over an input voltage range of 4V to 140V,
ideal for industrial, telecom, automotive and other
applications subject to harsh line transients. Due to
the regulator’s hysteretic architecture, no external
compensation is required. The output voltage is pinprogrammable to 1.8V, 3.3V or 5.0V, or if an external
divider is used, the output can be adjusted from 0.8V
to VIN. 100% duty cycle operation is possible because
of the internal P-channel FET. The LTC7138 is offered
in a thermally enhanced high voltage (skipped pins)
MSOP package.
Simple 5V/400mA Buck with Wide VIN Range
Figure 1 shows a 5V buck converter with a maximum
input voltage of 140V using only four external
components. Its output is set to 5V by tying the VPRG1
pin to the SS pin and the VPRG2 pin to ground. It does
not require any external compensation.
Efficiency remains high over a broad range of loads
due in part to the LTC7138’s Burst Mode® operation
and 12µA no load IQ current. For a 12V input, efficiency
peaks at 87% for a load of 10mA and stays above
80% for loads down to 0.4mA. This level of energy
conservation makes it ideal for always-on battery
operated systems.
24V/800mA Buck Using Two LTC7138s in Parallel
Figure 3 shows two LTC7138s paralleled to provide
an output of 24V at 800mA. Parallel operation is
implemented by tying the feedback comparator output
pin (FBO) of the master to the VFB pin of the slave. In
this setup, the slave follows the master as it enters
and exits the burst cycles (see Figure 4). The output of
the 24V buck is set with an external feedback divider.
The full load efficiency for this regulator is 93.6% for
a 48V input (Figure 5).
32V/400mA Surge Stopper
Figure 6 shows another use for the LTC7138. For inputs
of 32V or below, this regulator operates in dropout,
where the internal power FET is on continuously. When
the input exceeds 32V, the LTC7138 switches to keep
the output voltage in regulation, as shown in Figure 7.
For further protection, an overvoltage lockout can be
implemented by tying the OVLO pin to a divider across
the input supply. This circuit is suitable for protecting
downstream converters or loads in industrial,
automotive or avionics systems.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
100
90
L: SUMIDA CDRH105RNP-151NC
VIN
5V TO 140V C
IN
1µF
250V
VIN
SW
LTC7138
ANODE
RUN
SS
VPRG1
VFB
OVLO
VPRG2
GND
D1
VOUT
5V
COUT 400mA
22µF
EFFICIENCY (%)
80
L1
150µH
70
60
50
40
30
10
0
DN543 F01
Figure 1. High Efficiency 5V, 400mA Buck Regulator
10/15/543
VIN = 12V
VIN = 48V
VIN = 100V
VIN = 140V
20
0.1
1
10
ILOAD (mA)
100
1k
DN543 F02
Figure 2. Efficiency of the Regulator in Figure 1
More Features
The LTC7138 provides additional features which
makes it suitable for a wide array of applications.
These include a RUN pin for an external UVLO, an
ILIM pin for programming the current limit or setting
up an input side current limit, and soft-start —either
internal or external.
Conclusion
The LTC7138 yields low parts count, rugged solutions
for wide input voltage applications. It features a 140V
maximum input voltage rating, thermally enhanced
high voltage MSOP package, Burst Mode operation, low
IQ current, pin adjustable output voltage, no external
compensation and operation at 100% duty cycle.
100
90
L1
220µH
SW
LTC7138
(MASTER)
RUN
ANODE
VOUT
24V
800mA
VIN
CIN1
1µF
250V
X7R
OVLO
VFB
SS
VPRG1
VPRG2
ISET
FBO
D1 732k
80
EFFICIENCY (%)
VIN
24V TO 140V
COUT
10µF
6x
200k
GND
70
60
50
40
20
10
0
L2
220µH
SW
LTC7138
(SLAVE)
VFB
RUN
VIN
CIN2
1µF
250V
X7R
OVLO
ANODE
VPRG2
VPRG1
ISET
FBO
SS
VIN = 36V
VIN = 48V
VIN = 100V
VIN = 140V
30
D2
L: COILCRAFT MSS1278-224KLB
1
0.1
100
1k
DN543 F05
Figure 5. Efficiency of the Regulator in Figure 3
L1
100µH
VIN
4V TO 140V
VIN
RUN
FBO
SS
VPRG1
VPRG2
CIN
1µF
250V
X7R
L1/L2: COILCRAFT MSS1278-224KLB
D1/D2: DIODES INC SBR1U200P1-7
COUT : TAIYO YUDEN UMK325BJ106MMHT
VOUT*
400mA
SW
LTC7138
GND
DN543 F03
10
ILOAD (mA)
D1
ANODE
VFB
ISET
OVLO
536k
100k
COUT
10µF
50V
X5R
GND
DN543 F06
Figure 3. High Efficiency 24V, 800mA Buck Using Two
LTC7138s in Parallel
L1: SUMIDA CDRH104RNP-101NC
COUT : TAIYO YUDEN UMK325BJ106MMHT
D1: DIODES INC SBR1U200P1-7
* WHEN VIN > 32V, LTC7138 SWITCHES
AND VOUT IS REGULATED TO 32V;
WHEN VIN ≤ 32V, LTC7138 OPERATES
IN DROPOUT AND VOUT FOLLOWS VIN.
Figure 6. 32V, 400mA Surge Stopper
127V
IL1
500mA/DIV
VIN
50V/DIV
IL2
500mA/DIV
24V
32V CLAMP
VOUT
20V/DIV
VOUT
500mV/DIV
AC-COUPLED
LOAD = 400mA
20µs/DIV
DN543 F04
Figure 4. Parallel Operation of the
24V, 800mA Buck at VIN = 140V,
IOUT = 600mA
Data Sheet Download
www.linear.com/LTC7138
Linear Technology Corporation
20ms/DIV
DN543 F07
Figure 7. Output of the Surge
Stopper (Figure 6) Is Clamped to
32V During 127V Input Transient
For applications help,
call (408) 432-1900, Ext. 3720
dn543f LT/AP 1015 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2015
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