DEMO MANUAL DC2123A
LT3790
60V Four-Switch Synchronous
Buck-Boost Controller
DESCRIPTION
The DC2123A is a 60V four-switch synchronous buck-boost
controller that demonstrates the high power capability
of the
LT
®
3790.
The output is 24V and the maximum
output current is 5A. The switching frequency is 200kHz
and efficiency is as high as 98% for a 24V input.
The LT3790 is capable of high power operation. DC2123A
can be altered for either lower input voltage operation with
the same output power or for increased output power
with higher minimum input voltage. V
IN
can be reduced to
8V
IN
minimum and still deliver 120W output by changing
RS1 to 2mΩ, RIN1 to 1.5mΩ, and R3 (UVLO) to 88.7k. If
V
IN
remains always above 20V, I
OUT
can be increased to
12A
+
(for 290W output) with the above changes, and an
additional change of ROUT to 4mΩ.
The operating input voltage range of DC2123A is from
12V to 57V. The output voltage, EN/UVLO, and OVLO are
all programmed by resistor dividers. EN/UVLO is set so
the circuit will turn off when the input voltage falls below
11.9V and will turn on when the input voltage rises above
13.4V. OVLO is set to engage for input voltages above 57V.
Current sense resistors program input and output current
limits and also determine the monitoring voltages that
indicate input and output current. IVINMON provides a
60mV/1A voltage that is used to monitor the input cur-
rent. Input current limit occurs at 16.7A and is set by
RIN1. ISMON provides a 160mV/1A voltage that is used
to monitor the output current. The output current limit is
7.5A and is set by RS2.
The demo circuit features MOSFETs that compliment the
5V gate drive of the LT3790 to achieve high efficiency. 60V
MOSFETs are used on the input side of the four-switch
topology while 40V MOSFETs are used on the output side.
Ceramic capacitors are used at both the circuit input and
output because of their small size and high ripple current
capability. In addition to ceramic capacitors, there is an
aluminum polymer capacitor on the output that assures
feedback loop stability, even at low temperatures. The
input has an aluminum electrolytic capacitor in addition
to ceramic capacitors. A two-stage L-C input filter can
easily be added for electro-magnetic compatibility. The
pcb has large copper planes and extensive vias for thermal
performance.
The CLKOUT output and the SYNC input can be used to
synchronize switching between two or more DC2123A
circuits. A resistor from SYNC to ground must be removed
prior to using the SYNC input.
SHORT
and
C/10
are open-collector status flag outputs and
are pulled up to the INTV
CC
pin voltage. A resistor shorts
the CCM pin to
C/10
and causes the circuit to change to
discontinuous conduction mode (DCM) when
C/10
is active
at light load currents. CCM can also be connected with a
resistor to INTV
CC
instead of
C/10
for continuous conduc-
tion mode (CCM) operation over the entire load range.
The CTRL input is pulled up to the V
REF
pin through a 0Ω
resistor to set the output current limit to its maximum,
and an external voltage on CTRL can be used to lower the
current limit if the resistor is removed. A capacitor at the
SS pin programs soft-start and additionally SS is pulled
up to the V
REF
pin through a 100k resistor. The switching
frequency is adjustable with a resistor.
The demo circuit is designed to be easily reconfigured to
many other applications, including the example schemat-
ics in the data sheet. Consult the factory for assistance.
High power operation, four-switch buck-boost topology,
fault protection and full monitoring make the LT3790
attractive for high power voltage regulator circuits and
also circuits that require output current regulation such
as battery chargers. The LT3790EFE is available in a
thermally enhanced 38-lead TSSOP package. The LT3790
data sheet must be read in conjunction with this demo
manual to properly use or modify demo circuit DC2123A.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2123A
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
dc2123af
1
DEMO MANUAL DC2123A
PERFORMANCE SUMMARY
PARAMETER
Minimum Input Voltage, V
PVIN
Maximum Input Voltage, V
PVIN
Maximum Output Current
Input EN Voltage, V
PVIN(EN)
Input UVLO Voltage, V
PVIN(UVLO)
Input OVLO Turn-Off Voltage, V
PVIN(OVLO turn-off)
Input OVLO Turn-On Voltage, V
PVIN(OVLO turn-on)
Output Voltage V
OUT
Efficiency
Switching Frequency
Input Current Limit
Output Current Limit
Specifications are at T
A
= 25°C
CONDITIONS
V
OUT
= 24V I
OUT
≤ 4.5A
V
OUT
= 24V I
OUT
≤ 5A
13.3V < V
PVIN
< 57V, CTRL = V
REF
V
PVIN
= 12V, CTRL = V
REF
R3 = 56.2k, R4 = 499k, V
PVIN
Rising
R3 = 56.2k, R4 = 499k, V
PVIN
Falling
R11 = 27.4k, R1 = 499k, V
PVIN
Rising
R11 = 27.4k, R1 = 499k, V
PVIN
Falling
R19 = 71.5k, R34 = 1.37k, R20 = 3.83k
V
PVIN
= 24V, I
OUT
= 5A
R18 = 147k
RIN1 = 0.003Ω
RS2 = 0.008Ω
23.5
98
200
16.7
7.5
5
4.5
13.4
11.9
57.6
56.2
24.5
MIN
TYP
12
57
MAX
UNITS
V
V
A
A
V
V
V
V
V
%
kHz
A
A
QUICK START PROCEDURE
It is easy to set up demonstration circuit DC2123A to
evaluate the performance of the LT3790. Refer to Figure 1
for the proper measurement equipment setup and follow
the procedure below:
NOTE: Make sure that the voltage applied to PV
IN
does
not exceed the absolute maximum voltage rating of 60V
for the LT3790.
1. Connect the EN/UVLO terminal to ground with a clip-on
lead. Connect the power supply (with power off), load,
and meters as shown.
2. After all connections are made, turn on the input power
and verify that the input voltage is between 12V and 57V.
3. Remove the clip-on lead from EN/UVLO. Verify that the
output voltage is 24V.
NOTE: If the output voltage is low, temporarily disconnect
the load to make sure that it is not set too high.
4. Once the proper output voltage is established, adjust
the input voltage and load within the operating ranges
and observe the output voltage regulation, ripple volt-
age, efficiency and other parameters.
2
dc2123af
DEMO MANUAL DC2123A
QUICK START PROCEDURE
30
25
20
V
OUT
(V)
15
10
5
0
0
1
2
3
4
I
OUT
(A)
5
6
7
8
dc2009a F05
Figure 5. V
OUT
in Current Limit, V
IN
= 24V
DIFFERENCES BETWEEN LT3790 AND LT3791-1
The LT3790 is an improved version of the LT3791-1 and
is recommended for use in new designs. Some external
component values may change, but otherwise, the LT3790
is functionally equivalent to the LT3791-1. The differences
between the two products are:
1. The LT3790 has a 60mV (typical) full-scale V
(ISP-ISN)
current sense voltage, compared to 100mV (typical) for
the LT3791-1. This change allows lower power current
sense resistors to be used for most applications.
2. The LT3790 CTRL pin linear range is from 0V to 1.1V,
and has a turn-off threshold of 50mV(typical), compared
to a 200mV to 1.1V linear range and 175mV (typical)
turn-off threshold for the LT3791-1. These changes
make it easier to parallel two or more LT3790 ICs for
higher power levels.
3. The LT3790
C/10
pin pulls low when the V
(ISP-ISN)
voltage
is less than 1/10 full scale, compared to the LT3791-1,
where
C/10
pulls low when both V
(ISP-ISN)
is less than
1/10 full scale and V
FB
is greater than 1.15V(typical).
Since the
C/10
pin is used to allow DCM mode for some
applications, this change ensures that negative current
does not occur at light loads for a broader range of
applications.
dc2123af
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