LTC3725 Datasheet, PDF (15/20 Pages) Linear Technology – Single-Switch Forward Controller and Gate Driver

APPLICATIO S I FOR ATIO
V_IN+
V_BIAS
V_OUT+
V_IN NDRV V_CC
NDRV
UVLO FB/IN⁺
V_CC
FS/IN^–
SS/FLT
LTC3725
(MASTER)
••
FS/SYNC
PT⁺
FB
PT^–
ITH
RUN/SS
LTC3706
(MASTER)
V_IN–
LTC3725
NDRV
SS/FLT FB/IN⁺
V_CC
UVLO FS/IN^–
LTC3725
(SLAVE)
••
V_IN NDRV V_CC
RUN/SS FS/SYNC
PT⁺
FB
ITH
PT^–
PHASE
LTC3706
(SLAVE)
3725 F04
Figure 4. Connections for PolyPhase
Standalone Primary-Side Operation
The LTC3725 can be used to implement a standalone
forward converter using optoisolator feedback and a
secondary-side voltage reference. Alternately the LTC3725
can be used to implement an open-loop forward converter
using the V_SLMT pin to regulate against changes in V_IN. In
either case, the LTC3725 oscillator determines the fre-
quency as found from:
f_OSC
=
21• 10⁹
R_FS(P) + 4200
Note that polyphase operation is not possible in the stand-
alone configuration.
Grounding Considerations
The LT3725 is typically used in high current converter
designs that involve substantial switching transients. Fig-
ure 5 illustrates these currents. The switch driver on the IC
is designed to drive a large capacitance and, as such,
generate significant transient currents. Careful consider-
ation must be made regarding input and local power
supply bypassing to avoid corrupting the ground refer-
ences used by the UVLO and frequency set circuitry.
Typically, high current paths and transients from the input
supply and any local drive supplies must be kept isolated
from GND. By virtue of the topologies used in LT3725
applications, the large currents from the primary switch,
as well as the switch drive transients, pass through the
sense resistor to ground. This defines the ground connec-
tion of the sense resistor as the reference point for both
GND and PGND.
3725f
15