Linear Technology LTC3725 Quick Start Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 888A-B
DESCRIPTION
Demonstration circuit 888A-B is a high power iso-
lated synchronous forward converter featuring the
LTC3725 and LTC3706. When powered from a 36-
72V input, a single DC888A-B provides an isolated
5V at 40A in a quarter-brick footprint. If higher out-
put current is required, multiple DC888A boards may
be stacked together using on-board connectors for a
complete PolyPhase current sharing solution. The
converter operates at 250kHz and achieves efficiency
up to 93.5% with synchronous output rectifiers.
Secondary-side control eliminates complex optocou-
Table 1. Performance Summary (T
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT
Maximum Output Current
Typical Output Ripple V
OUT
Output Regulation
Load Transient Response
Nominal Switching Frequency
Efficiency
Isolation
Size
OPERATING PRINCIPLES – SINGLE PHASE
The LTC3706 secondary side controller is used on the
secondary and the LTC3725 smart driver with self-
starting capability is used on the primary. When an
input voltage is applied, the LTC3725 (U1 in Figure
15), which is powered through R29 and Q28, begins
a controlled soft-start of the output voltage by switch-
ing MOSFETs Q9 and Q11. As the output voltage be-
gins to rise, the LTC3706 secondary controller is
quickly powered up via D24, Q29, C67, and Q27. The
36V-72VIN, ISOLATED SYNCHRONOUS FORWARD
= 25°C)
A
CONDITION
V = 36V to 72V, I
IN
200LFM
V = 48V, I = 40A, 250kHz
IN OUT
Over All Input Voltages and Output Currents
Peak Deviation with 20A to 40A Load Step (10A/us)
Settling Time
V = 36V, I = 28A
IN OUT
BASIC
Component Area x Top x Bottom Component Height
pler feedback, providing fast transient response with
a minimum amount of output capacitance. Additional
DC888A versions include DC888A-A (3.3V at 50A)
and DC888A-C (12V at 20A). The simple architecture
can be easily modified to meet different input and
output voltage requirements.
Design files for this circuit board are available.
Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
= 0A to 40A
OUT
LTC3706 then assumes control of the output voltage
by sending encoded PWM gate pulses to the
LTC3725 primary driver via signal transformer, T2.
The LTC3725 then operates as a simple driver receiv-
ing both input signals and bias power through T2.
The transition from primary to secondary control oc-
curs seamlessly at a fraction of the output voltage.
From that point on, operation and design simplifies to
that of a simple buck converter. The LTC3706 regu-
LTC3725/LTC3706
VALUE
36V
72V
5V
40A
< 60mV
P–P
±1% (Reference)
±250mV
•50us
250kHz
93.5% Typical
1500VDC
2.3" x 1.45" x 0.4" x 0.075"
1