DC423 Quick Start Guide Datasheet by Analog Devices Inc.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 423
1A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
LT1956
DESCRIPTION
Demonstration circuit 423 is a 60V, 500kHz, 1A load cur-
rent, monolithic step-down DC/DC switching converter
using the LT1956. With its wide input voltage range,
1.5A internal power switch, and thermally enhanced
package, the LT1956 is a very versatile and powerful IC
for DC/DC converters that require compact space and a
wide variety of input voltage ranges. The constant
500kHz switching frequency allows all of the compo-
nents to be small, surface mount devices. The current-
mode control topology creates fast transient response
and good loop stability with a minimum number of ex-
ternal compensation components and the use of ceramic
input and output capacitors. The low resistance internal
power switch (0.2
Ω
) maintains high efficiencies >75%
and up to 90%. The SHDN pin and 25
µ
A shutdown cur-
rent extends battery life. The wide VIN range of the
LT1956 allows steady state input voltages from 7.5V
input up to 25V input. The LT1956 is capable of steady
state operation of up to 60V if the output voltage is set
higher than 5V and the duty cycle remains high enough
(typically above 25%). Minimum on-time and maximum
output power at high input voltages may have thermal
restrictions (please see datasheet for more details).
However, with 5V output, the LT1956 is designed for
transient operation up to 60V. Synchronization of switch-
ing frequencies up to 700kHz is possible.
Demonstration Circuit 423 is designed for 7.5V to 25V
input (with up to 60V transient) to 5.0V output at 1A
load current for automotive, industrial, portable com-
puters, battery chargers, and distributed power systems.
This board was designed for wide input voltage range
applications where simplicity, small circuit size, and low
component count are important as well as up to 1A of
load current. The extremely low output voltage ripple
ceramic capacitor not only demonstrates small size and
low cost, but the advantage of current-mode control in
step-down applications with a simple compensation
network and a feedforward capacitor for more rugged
stability and excellent transient response. Demonstration
Circuit 423 is best used in 500kHz applications where
the maximum duty cycle does not exceed 75% and the
minimum steady state duty cycle remains above 25%.
Duty cycles below 25% are capable, with the restriction
of minimum on-time, the part may begin to skip pulses
to effectively lower the overall minimum on-time.
Design files for this circuit board are available. Call
the LTC factory.
Table 1.
Typical Performance Summary
PARAMETER CONDITION VALUE
Steady State Input Voltage Range V
OUT
= 5V, I
OUT
≤
1A 7.5–25V
Maximum Transient Input Voltage 60V
V
OUT
I
OUT
≤
1A 5V ±3%
Maximum Output Current 1A
Typical Switching Frequency 500kHz
Typical Efficiency V
IN
= 12V, V
OUT
= 5V, I
OUT
= 1A 83%
1
49—“?
1.7 L'HW
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 423
1A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
QUICK START PROCEDURE
Demonstration circuit 423 is easy to set up to evaluate
the performance of the LT1956. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
Make sure that the input voltage does not exceed 60V.
The synchronization and shutdown functions are op-
tional and their pins can be left floating (disconnected) if
their functions are not being used.
1.
Connect the 7.5V–25V input power supply to the Vin
and Gnd terminals on the board.
2.
After all connections are made, turn on input power
and verify that the output voltage is 5.0V.
LOAD
+–
+–
+–
+–
+–+–
+–
OPTIONAL
OPTIONAL
Figure 1.
Proper Measurement Equipment Setup
CUSTOMIZING THE BOARD
The components assembled on the board are optimized
for a wide input voltage range and a 5V output. The
bandwidth can be increased for more specific input volt-
ages such as 12V ±10% or 18V–24V with 36V transient
if desired (just a few examples). The adjustable feedback
resistors allow the output voltage to be customized. For
output voltages below 3.0V, the boost diode should be
moved from D2 to D3 to provide the minimum boost
voltage required for the internal power switch. Make
sure that the boost capacitor (C8) has a voltage rating
greater than or equal to the output voltage for applica-
tions where the boost diode is placed in D2. However,
the boost capacitor must have a voltage rating greater
than the input voltage whenever the boost diode is
placed in position D3.
D4 is an optional boost voltage “blocking” diode. For
custom applications in which the output voltage is sig-
nificantly greater than 5V, this blocking diode reduces
the boost driver voltage, reducing the power (and thus
heat) dissipation inside the IC. It is recommended that a
CMHZ5236B zener diode is used in D4 when V
OUT
= 12V
and the junction temperature is hot without the use of
the Zener. Note that in order to properly install D4, the
small trace connecting the anode and cathode of D4 on
the board must be separated (an Exacto knife works
well) before D4 is soldered to the board.
0
10
20
30
40
50
60
70
80
90
100
0 200 400 600 800 1k
LOAD CURRENT (mA)
EFFICIENCY (%)
8VIN
12VIN
18VIN
24VIN
Figure 2.
Efficiency
2
1.7 L'HW
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 423
1A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
SHDN
SYNC
GND
GND
5V@1A
7V-25V
1. ALL RESISTORS ARE IN OHMS, 0603.
ALL CAPACITORS ARE IN MICROFARADS, 0603.
NOTE: UNLESS OTHERWISE SPECIFIED
VIN
VOUT
2. ALL OPTIONAL COMPONENTS WILL HAVE NO
COMPONENT OUTLINE ON BOARD EXCEPT Q1,
R5, R6, AND C9.
*
*
HIGHER D.C. INPUT VOLTAGES ALLOWED UP TO 60V
(SEE QUICK START GUIDE)
VC
E6
+
C10
OPT
R6
OPT
R1
4.7k C1
220pF
R4
OPT
R2
15.4k
1%
C5
22uF,6.3V
1210
L1
15uH
CDRH6D28
SUMIDA
1 2
D1
10MQ060N
SMA
2 1
C2
4700pF
E5
C8
OPT
R3
4.99k
1%
C9
OPT
E2
R5
OPT
D3
OPT
MMSD914
2 1
D2
MMSD914
SOD123
21
U1
LT1956EFE
3 4
15
14
7
13
1
8
916 11
12
10
5
2
6
NC VIN
SHDN
SYNC
NC
NC
GND
GND GND GND VC
FB
BIAS
NC
SW
BOOST
Q1
OPT
MMBT2222A
1
2 3
E3
D4
OPT
CMHZ5236B
2 1
C3
2.2uF
100V
E4
C4
0.1uF
16V
C6
OPT
0805
0.47uF, 25V
E1
Figure 3.
Demo Circuit Schematic
3
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