LOW QUIESCENT CURRENT, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1352A
LTC3857EGN-1/LTC3858EGN-1
DESCRIPTION
Demonstration circuit 1352A is a Low Quiescent
Current, Dual Output Synchronous Buck Conver-
ter featuring the LTC3857EGN-1/LTC3858EGN-1.
The circuit is single sided layout, while the pack-
age style for the LTC3857/8EGN-1 is a 28-lead
narrow plastic SSOP.
The main features of the board include rail track-
ing (LTC3857EGN-1 only), an internal 5V linear
regulator for bias, RUN pins for each output, a
PGOOD signal (CH1 only) and a Mode selector
that allow the converter to run in CCM, pulse skip
or Burst Mode operation. Synchronization to an
external clock is also possible.
Two versions of the board are available. DC-
1352A-A is for the LTC3857EGN-1, while the
DC1352A-B is for the LTC3858EGN-1. The
Table 1.
Performance Summary (T
A
= 25°C)
CONDITION
VALUE
4.5V
#
36V
V
IN
= 4.5V to 36V, I
OUT1
= 0A to 5A
V
IN
= 9V to 36V, I
OUT2
= 0A to 3A
3.3V ±2%
8.5V ±2%
350kHz
V
IN
= 24V
V
OUT1
= 3.3V, I
OUT1
= 5A
V
OUT2
= 8.5V, I
OUT1
= 3A
* Measured at bulk output capacitor
#
LTC3857EGN-1 offers lower quiescent current
and smaller burst mode ripple, while the
LTC3858EGN-1 offers latch-off protection and in-
creased burst mode efficiency.
The wide input voltage range of 4.5V to 36V is
suitable for automotive or other battery fed appli-
cation where low quiescent current is important.
The LT3857EGN-1 and LTC3858EGN-1 data-
sheets give a complete description of these parts,
operation and application information. The data-
sheets must be read in conjunction with this quick
start guide for demo circuit 1352A.
Design files for this circuit board are available.
Call the LTC factory.
Burst Mode is a trademark of Linear Technology Corporation
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT1
Output Voltage V
OUT2
Nominal Switching Frequency
Efficiency, DC1352A-A/B
See Figures 3 to 6 for efficiency curves
V
IN
= 36V
90.4%* Typical
95.4%* Typical
92.5%* Typical
96.6%* Typical
Minimum input voltage required for 8.5Vout regulation is 9Vin
QUICK START PROCEDURE
Demonstration circuit 1352A is easy to set up to
evaluate the performance of the LTC3857EGN-
1/LTC3858EGN-1. Refer to Figure 1 for proper
measurement equipment setup and follow the
procedure below:
NOTE:
When measuring the input or output voltage
lead on the oscilloscope probe. Measure the input
or output voltage ripple by probing directly across
the bulk Vin or Vout capacitor. See Figure 2 for
proper scope probe technique.
1.
Place jumpers in the following positions:
ripple, care must be taken to avoid a long ground
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LOW QUIESCENT CURRENT, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
JP2
JP3
JP4
JP5
JP6
Burst Mode
On
On
SS
SS
4.
Check for the proper output voltages.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1352A
Vout1 = 3.234V to 3.366V,
Vout2 = 8.330V to 8.670V
NOTE:
If there is no output, temporarily discon-
nect the load to make sure that the load is not
set too high.
5.
Once the proper output voltages are estab-
2.
With power off, connect the input power supply
to Vin and GND.
3.
Turn on the power at the input.
NOTE:
Make sure that the input voltage does not
lished, adjust the loads within the operating
range and observe the output voltage regula-
tion, ripple voltage, efficiency and other parame-
ters.
6.
Different operating modes can be evaluated by
exceed 26V.
Iout1
+
changing position of jumper JP3.
A
Vout1
Vout1
load
-
-
V
+
Iin
A
+
Vin
+
V
-
Vin supply
-
-
Vout2
load
+
-
A
Iout2
V
Vout2
+
Figure 1. Proper Measurement Equipment Setup
2
LOW QUIESCENT CURRENT, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1352A
Figure 2. Measuring Input or Output Ripple Directly Across Bulk Capacitor
RAIL TRACKING
Demonstration circuit 1352 is setup for indepen-
dent soft start without tracking. The soft start
ramp-rate is determined by the value of the SS
capacitors C1 and C23. This board can also be
operated in coincident tracking mode with either
Table 2.
Output Tracking Options
TRACK/SS JUMPERS
CONFIGURATION
Soft Start Without Tracking
Vout1
Vout2
Coincident Tracking:
Vout1 tracking External Ramp
Vout2 tracking External Ramp
Vout2 tracking Vout1
Vout1 tracking Vout2
SS
TRACK
TRACK
TRACK
TRACK
SS
No connection / Don’t care
Connect to Vout2
Connect external ramp
Connect external ramp
Connect to Vout1
No connection / Don’t care
SS
SS
No connection / Don’t care
No connection / Don’t care
JP3
JP4
TRACK1
TERMINALS
TRACK2
output as master or both can slave an external
ramp Refer to Table 2 for tracking options and to
the data sheet for more details.
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LOW QUIESCENT CURRENT, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1352A
INDUCTOR DCR SENSING AND RESISTOR SENSING
The DCR sense circuit uses the resistive voltage
drop across the inductor to estimate the current.
In contrast to the traditional sense resistor cur-
rent feedback, the DCR sensing circuit offers
lower cost and higher efficiency, but results in
less accurate current limit due to the large varia-
tion of the inductor DC resistance. Furthermore,
this indirect current sensing method cannot
detect inductor saturation and requires the use
of ‘soft’ saturating inductors (such as powder
Table 3.
Vout1
Vout2
iron) resulting in increased core losses or ‘hard’
saturating inductors (such as ferrite) with suffi-
ciency high current ratings resulting in increased
inductor size.
For modifying the demo board for DCR sensing,
please refer to Table 3. A full load efficiency im-
provement of between 0.25% – 0.75% is still
possible with optional DCR sensing, but since
the inductors are ferrite based, short circuit per-
formance may be compromised.
DCR sensing component selection
REMOVE RSENSE NETWORK
R8, R9 = Open, R10 = Short
R20, R21 = Open, R17 = Short
R28 = 1.37kΩ
R32 = 1.91kΩ
DCR NETWORK
R30 = 2.74kΩ
R31 = 1.54kΩ
R29 = 0Ω
R33 = 0Ω
C6 = 0.47µF
C18 = 0.47µF
FREQUENCY SYNCHRONIZATION
Demonstration circuit 1352’s Mode selector al-
lows the converter to run in CCM, pulse skip or
Burst Mode operation by changing position of
jumper JP2. For synchronizing to an external
clock source, remove jumper JP2 entirely and
apply the external clock signal to the
PLLIN/MODE pin. Please refer to datasheet for
details on external clock signal requirements.
4
LOW QUIESCENT CURRENT, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
LTC3857EGN-1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1352A
Figure 3. Typical Efficiency vs. Load Current for A-A board – 24Vin, 3.3Vout and 8.5Vout
LTC3857EGN-1
Figure 4. Typical Efficiency vs. Load Current for A-A board – 36Vin, 3.3Vout and 8.5Vout
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