DN512 - 65V, 500mA Step-Down Converter Fits Easily into Automotive and Industrial Applications

65V, 500mA Step-Down Converter Fits Easily into Automotive
and Industrial Applications
Design Note 512
Charlie Zhao
VIN
48V
100µH
VIN
+
OPT.
2M
47µF
RUN
FBO
68.1k
ISET
200pF
100k
806k
VFB
VPRG1
VIN = 30V
EFFICIENCY (%)
96
92
VIN = 48V
88
84
80
76
1
10
100
LOAD CURRENT (mA)
1000
F02
Figure 2. Efficiency of Circuit in Figure 1
VIN
35
30
27V
25
20
24V
30
VOUT
20
VOUT
24V, 300mA
2× 22µF
SW
LTC3630
100
10
0
221k
SS
47nF
VPRG2
GND
TIME (5s/DIV)
OUTPUT VOLTAGE (V)
65V Input, 500mA DC/DC Converter with an
Adjustable Output Down to 800mV
The LTC ®3630 is a versatile Burst Mode ® synchronous
step-down DC/DC converter that includes three pinselectable preset output voltages. Alternatively, the
output can be set via feedback resistors down to 800mV.
An adjustable output or input current limit from 50mA
to 500mA can be set via a single resistor. The hysteretic
nature of this topology provides inherent short-circuit
protection. Higher output currents are possible by paralleling multiple LTC3630s together and connecting the
FBO of the master device to the VFB pin of a slave device.
An adjustable soft-start is included. A precision RUN
pin threshold voltage can be used for an undervoltage
lockout function.
24V Regulator with 300mA Output Current Limit
and Input Undervoltage Lockout
Figure 1 shows a 48V to 24V application that showcases
several of the LTC3630’s features, including the undervoltage lockout and output current limit. Operational
efficiencies are shown in Figure 2.
INPUT VOLTAGE (V)
Introduction
The trend in automobiles and industrial systems is to
replace mechanical functions with electronics, thus multiplying the number of microcontrollers, signal processors,
sensors and other electronic devices throughout. The issue
is that 24V truck electrical systems and industrial equipment use relatively high voltages for motors and solenoids
while the microcontrollers and other electronics require
much lower voltages. As a result, there is a clear need for
compact, high efficiency step-down converters that can
produce very low voltages from the high input voltages.
F03
Figure 3. Input Voltage Sweep vs Output Voltage
Showing Undervoltage Lockout Threshold Levels
F01
Figure 1. High Efficiency 24V Regulator with
Undervoltage Lockout and 300mA Current Limit
03/13/512
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.
VIN = 48V
25
80Ω
OUTPUT
VOLTAGE
OUTPUT
CURRENT
300
15
42Ω
10
200
25Ω
5
RESISTIVE
LOAD
0
18Ω
14Ω 11Ω
100
8Ω
0
OUTPUT CURRENT (mA)
VOUT (V)
20
VIN
10V TO 60V
100µH
+
VIN
47µF
OPT.
665k
LTC3630
RUN
FBO
ISET
100nF
VOUT
5V
2× 22µF
SW
VFB
VPRG1
SS
47nF
10k
VPRG2
GND
F05
F04
Figure 4. Resistive Load Sweep vs Output Current vs
Output Voltage with Output Current Limit Set to 300mA
Figure 5. 5V Regulator with 55mA Input Current Limit
The RUN pin is programmed for VIN undervoltage
lockout threshold levels of 27V rising and 24V falling.
Figure 3 shows VOUT vs VIN. This feature assures that
VOUT is in regulation only when sufficient input voltage
is available.
An increased voltage on ISET increases the converter’s
current limit. Figure 6 shows the steady-state input
current vs input voltage and the available output current
before the output voltage begins to drop out of regulation. For the values shown in Figure 5, the input current
is limited to approximately 55mA over a 10V to 60V input
voltage range.
Although the LTC3630 can supply up to 500mA of output
current, the circuit in Figure 1 is programmed for a maximum of 300mA. An internally generated 5µA bias out of
the ISET pin produces a voltage across an ISET resistor,
which determines the maximum output current. Figure 4
shows the output voltage as a resistive load is varied from
approximately 100Ω down to 8Ω while maintaining the
output current near the programmed value of 300mA.
In addition, the hysteretic topology used in this DC/DC
converter provides inherent short-circuit protection.
Input Current Limit
Another useful feature of the LTC3630 is shown in
Figure 5. In this 5V circuit, the current limit is set by a
resistive divider from VIN to ISET, which produces a voltage
on the ISET pin that tracks VIN. This allows VIN to control
output current which determines input current.
Data Sheet Download
www.linear.com/3630
Linear Technology Corporation
500
VOUT = 5V
400
CURRENT (mA)
The 24V output voltage can be programmed using the
800mV 1% reference or one of the preset voltages. This
circuit uses the 5V preset option along with feedback
resistors to program the output voltage. This increases
circuit noise immunity and allows lower value feedback
resistors to be used.
MAX LOAD
CURRENT BEFORE
VOUT DROPS
300
200
100
0
INPUT CURRENT
0
10
20
30
40
INPUT VOLTAGE (V)
50
60
F06
Figure 6. Input Voltage vs Load Current and Input Current
with Input Current Limit Circuit Shown in Figure 5
Conclusion
The LTC3630 offers a mixture of features useful in high
efficiency, high voltage applications. Its wide output
voltage range, adjustable current capabilities and inherent short-circuit tolerant operation makes this DC/DC
converter an easy fit in demanding applications.
For applications help,
call (408) 432-1900, Ext. 3258
dn512 LT/AP 0313 196K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2013
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