DEMO MANUAL DC2422A-A,
DC2422A-B, DC2422A-C
LTC7812EUH
Wide V
IN
, Step-Up + Step-Down
DC/DC Converter
DESCRIPTION
Demonstration circuit DC2422A is a step-up + step-down
DC/DC converter featuring the
LTC
®
7812EUH,
a high per-
formance dual output (Boost + Buck) synchronous DC/
DC switching regulator controller. This board has three
different configurations:
n
n
DC2422A-A:
Regulated V
OUT
to 12V
DC2422A-B:
Quasi regulated V
OUT
between 10V and
16V
DC2422A-C:
Dual outputs: 12V (boost channel)
5V (buck channel)
The LTC7812 buck input is supplied from the boost output
for the DC2422A-A and DC2422A-B only. With this boost-
then-buck topology, the buck output maintains regulation
over the entire input range. The wide input voltage range
of 4V to 36V is suitable for automotive or other battery
fed applications where low quiescent current is important.
The DC2422A-B is designed such that V
OUT
is partially
regulated between 10V and 16V for higher efficiency
(Figure 4a), reduced power loss, and switching noise.
When V
IN
is between 10V and 16V, it operates in pass-
thru mode with the boost synchronous FET fully on and
the buck in drop-out with >99% duty cycle. When V
IN
is
below 10V, the boost is activated and when V
IN
is above
16V, the buck is activated as shown in Figure 4b.
The LT7812 data sheet gives a complete description of
the part, operation and application information and must
be read in conjunction with this quick start guide.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2422A
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.
n
This circuit enables a drop-in layout where the total circuit
area is only 1½" by 1". The LTC7812EUH is in a 32-lead
exposed pad QFN package.
The main features of the board include rail tracking (buck
channel only), an internal 5V linear regulator for bias,
separate RUN pins for each output and a Mode selector
that allows the converter to run in CCM, pulse skip or
Burst Mode
®
operation. Synchronization to an external
clock is also possible.
PERFORMANCE SUMMARY
SYMBOL PARAMETER
V
IN
V
OUT1
Input Supply Range
Output1 Voltage Range
Specifications are at T
A
= 25°C
M
IN
4
11.7
9.7
4.9
11.7
5**
12
350
93
95.4
95
94.8
97.2
95.8
95.8
94
91.9
TYP
12
MAX
36
12.3
16.3
5.1
12.3
UNITS
V
V
V
V
V
kHz
%
%
%
%
%
%
%
%
%
dc2422af
CONDITIONS
DC2422A-A: V
IN
= 4V* – 36V, I
OUT1
= 0A to 8A
†
DC2422A-B: V
IN
= 4V* – 36V, I
OUT1
= 0A to 8A
†
DC2422A-C: V
IN
= 4V* – 36V, I
OUT1
= 0A to 8A
†
V
BUS
f
SW
V
BUS
Voltage Range (DC2422A-C only)
Typical Free Running Switching Frequency
Efficiency
See Figures 3, 4, 5 and 6 for Efficiency
Curves
DC2422A-A: V
IN
= 8V, V
OUT1
= 12V, I
OUT1
= 8A
V
IN
= 12V, V
OUT1
= 12V, I
OUT1
= 8A
V
IN
= 36V, V
OUT1
= 12V, I
OUT1
= 8A
DC2422A-B: V
IN
= 8V, V
OUT1
= 10V, I
OUT1
= 8A
V
IN
= 12V, V
OUT1
= 12V, I
OUT1
= 8A
V
IN
= 36V, V
OUT1
= 16V, I
OUT1
= 6A
DC2422A-C: V
IN
= 8V, V
BUS
= 12V, Ibus = 8A
V
IN
= 12V, V
OUT1
= 5V, I
OUT1
= 8A
V
IN
= 36V, V
OUT1
= 5V, I
OUT1
= 8A
†
I
OUT1
starts to derate when V
IN
< 8V, *V
IN
needs to be >4.5V to start up, **V
OUT1
follows V
IN
when V
IN
< 5V.
1
DEMO MANUAL DC2422A-A,
DC2422A-B, DC2422A-C
QUICK START PROCEDURE
Demonstration circuit DC2422A is easy to set up to evaluate
the performance of the LTC7812EUH. Refer to Figure 1
for proper measurement equipment setup and follow the
procedure below:
NOTE: When measuring the input or output voltage ripple,
care must be taken to avoid a long ground lead on the
oscilloscope probe. Measure the input or output voltage
ripple by touching the probe tip directly across the V
IN
or V
OUT
and GND terminals or directly across relevant
capacitor. See Figure 2 for proper scope probe technique.
1. Place jumpers in the following positions:
JP1:
JP2:
JP3:
ON
ON
Burst Mode
BURST
3. Turn on the power at the input.
NOTE: Make sure that the input voltage is higher than
8V and does not exceed 36V.
4. Check for the proper output voltages for different boards.
DC2422A-A: V
OUT1
= 11.7V to 12.3V,
DC2422A-B: 9.7V < V
OUT1
< 16.3V,
DC2422A-C: V
OUT1
= 4.9V to 5.1V; V
BUS
= 11.7V to
12.3V.
NOTE: If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
5. Once the proper output voltages are established, adjust
the loads within the operating range and observe the
output voltage regulation, ripple voltage, efficiency and
other parameters.
2. With power off, connect the input power supply to V
IN
and GND.
–
+
I
OUT
I
OUT
–
+
LOAD
–
–
+
LOAD
+
I
IN
+
–
+
–
V
IN
SUPPLY
Figure 1. Proper Measurement Equipment Setup
dc2422af
2
DEMO MANUAL DC2422A-A,
DC2422A-B, DC2422A-C
QUICK START PROCEDURE
+
V
OUT
C
OUT
–
GND
Figure 2. Measuring Input or Output Ripple Directly Across Ceramic Capacitor
FREQUENCY SYNCHRONIZATION AND MODE SELECTION
Demonstration circuit DC2422A’s Mode selector allows
the converter to run in forced continuous operation, pulse
skip operation, Burst Mode operation or be synchronizing
to an external clock by changing the position of JP3.
Table 1. Mode Selection and Synchronized Operation Options
CONFIGURATION
Forced Continuous Operation
Pulse Skip Operation
Synchronized to Ext. Clock Applied to CLKIN Pin
Burst Mode Operation
JP3
FCM
PS/Sync
PS/Sync
Burst
RAIL TRACKING
Demonstration circuit DC2422A is configured for an on-
board soft-start circuit. The soft-start ramp rate can be
adjusted by changing the value of C47. Demonstration
Table 2. V
OUT1
Tracking Options
CONFIGURATION
Soft-Start without Tracking (Default)
V
OUT1
Tracking Scaled External Reference
R34
OPEN
R37
OPEN
C47
0.1µF
OPEN
TRK/SS1 CAP
OPEN
External Reference
circuit DC2422A can also be modified to track an external
reference. Refer to Table 2 for tracking options and to the
data sheet for more details.
Resistor Divider
OPTIONAL INDUCTOR DCR CURRENT SENSING
Demonstration circuit 2422A provides an optional circuit
for Inductor DCR Current Sensing. Inductor DCR Current
Sensing uses the DCR of the inductor to sense the inductor
current instead of discrete sense resistors. The advantages
of DCR sensing are lower cost, reduced board space and
higher efficiency, but the disadvantage is a less accurate
current limit. If DCR sensing is used, be sure to select an
inductor current with a sufficiently high saturation current
or use an iron powder type material.
Refer to Table 3 for Optional Inductor DCR Current Sensing
setup and to the data sheet for more details.
dc2422af
3
DEMO MANUAL DC2422A-A,
DC2422A-B, DC2422A-C
OPTIONAL INDUCTOR DCR CURRENT SENSING
Table 3. Optional Inductor DCR Current Sensing
CHANNEL1
CONFIGURATION
Current Sense Resistor (Default)
Inductor DCR Current Sensing
CHANNEL2
RS1
RS2
Ref. Sch.
0Ω Copper
R39
R80
Ref. Sch.
OPEN
R40
R81
Ref. Sch.
OPEN
C15
C56
Ref. Sch.
R51
R89
OPEN
R53
R90
OPEN
R62
R91
OPEN
0Ω
Calculated Value from Data Sheet
Low Quiescent Current Applications and Measurement
The typical quiescent current (I
Q
) of the LTC7812 controller
is 28µA in sleep mode as specified in the LTC7812 data
sheet. However, the input current of the DC2422A board
can be higher than this value because of additional circuit
outside of the IC. To reduce the total input current, large
value FB divider resistors should be used. In addition, some
of the optional pull-up resistors should be removed from
the board. Refer to Table 4 for the low input quiescent
current setup.
Table 4. Low Input Quiescent Current Configuration
Reference Designator
Function
Stuffing Option
R46
PGOOD1
OPEN
R92
OV2
OPEN
100
90
80
EFFICIENCY (%)
70
60
50
40
V
OUT1
= 12V EFFICIENCY
30
0.1
V
IN
= 8V BURST
V
IN
= 36V BURST
V
IN
= 8V FCM
V
IN
= 36V FCM
1
I
OUT
(A)
10
dc2422a F03
Figure 3. DC2422A-A V
OUT1
Typical Efficiency vs Load Current
100
90
80
EFFICIENCY (%)
70
60
50
40
30
0.1
1
I
OUT
(A)
V
IN
= 12V
V
IN
= 8V
V
IN
= 36V
10
dc2422a F04a
V
OUT1
V
OUT
= V
IN
V
BUCK
V
BOOST
PASS-THRU
WIRE MODE
BOOST
ACTIVE
10V
16V
BUCK
ACTIVE
dc2422a F04b
V
IN
Figure 4a. DC2422A-B V
OUT1
Typical FCM Efficiency vs Load Current
Figure 4b. DC2422A-B Partially Regulated V
OUT
dc2422af
4
DEMO MANUAL DC2422A-A,
DC2422A-B, DC2422A-C
OPTIONAL INDUCTOR DCR CURRENT SENSING
100
95
90
EFFICIENCY (%)
EFFICIENCY (%)
85
80
75
70
65
60
55
50
0.1
1
I
OUT
(A)
PULSE SKIP
BURST
FCM
10
dc2422a F05
V
IN
= 8V, V
BUS
= 12V EFFICIENCY
100
90
80
70
60
50
40
30
0.1
1
I
OUT
(A)
V
OUT1
= 5V EFFICIENCY
V
IN
= 8V BURST
V
IN
= 36V BURST
V
IN
= 8V FCM
V
IN
= 36V FCM
10
dc2422a F06
Figure 5. DC2422A-C V
BUS
Typical Efficiency vs Load Current
Figure 6. DC2422A-C V
OUT1
Typical Efficiency vs Load Current
20MHz BW
20MHz BW
20mV/DIV
20mV/DIV
5µs/DIV
dc2422 F07
5µs/DIV
dc2422 F08
Figure 7. DC2422A-A V
OUT1
Voltage Ripple at V
IN
= 8V
Figure 8. DC2422A-A V
OUT1
Voltage Ripple at V
IN
= 36V
20MHz BW
20MHz BW
20mV/DIV
50mV/DIV
200µs/DIV
dc2422 F09
200µs/DIV
dc2422 F10
Figure 9. DC2422A-A V
OUT1
2A – 4A – 2A
Load Transient at V
IN
= 8V
Figure 10. DC2422A-A V
OUT1
2A – 4A – 2A
Load Transient at V
IN
= 36V
dc2422af
5
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