LTC3722-1 LINER [Linear Technology], LTC3722-1 Datasheet - Page 11

• Manufacturer Part Number: LTC3722-1
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: LINER [Linear Technology]

OPERATIO
3) Internally generated drive signals with programmable
turn-off for current doubler synchronous rectifiers.
Benefit: eliminates external glue logic, drivers, optimal
timing for highest efficiency.
4) Programmable (single resistor) leading edge blanking.
Benefit: prevents spurious operation, reduces external
filtering required on CS.
5) Programmable (single resistor) slope compensation.
Benefit: eliminates external glue circuitry.
6) Optimized current mode control architecture.
Benefit: eliminates glue circuitry, less overshoot at start-
up, faster recovery from system faults.
7) Programmable system undervoltage lockout and hys-
teresis.
Benefit: provides an accurate turn-on voltage for power
supply and reduces external circuitry.
As a result, the LTC3722-1/LTC3722-2 makes the ZVS
topology feasible for a wider variety of applications, in-
cluding those at lower power levels.
The LTC3722-1/LTC3722-2 control four external power
switches in a full-bridge arrangement. The load on the
bridge is the primary winding of a power transformer. The
diagonal switches in the bridge connect the primary wind-
ing between the input voltage and ground every oscillator
cycle. The pair of switches that conduct are alternated by
an internal flip-flop in the LTC3722-1/LTC3722-2. Thus,
the voltage applied to the primary is reversed in polarity on
every switching cycle and each output drive signal is 1/2
the frequency of the oscillator. The on-time of each driver
signal is slightly less that 50%. The on-time overlap of the
diagonal switch pairs is controlled by the LTC3722-1/
LTC3722-2 phase modulation circuitry. (Refer to Block
and Timing Diagrams) This overlap sets the approximate
duty cycle of the converter. The LTC3722-1/LTC3722-2
driver output signals (OUTA to OUTF) are optimized for
interface with an external gate driver IC or buffer. External
power MOSFETs A and C require high side driver circuitry,
while B and D are ground referenced and E and F are
ground referenced but on the secondary side of the
U
isolation barrier. Methods for providing drive to these
elements are detailed in this data sheet. The secondary
voltage of the transformer is the primary voltage divided
by the transformer turns ratio. Similar to a buck converter,
the secondary square wave is applied to an output filter
inductor and capacitor to produce a well regulated DC
output voltage.
Switching Transitions
The phase shifted full-bridge can be described by four
primary operating states. The key to understanding how
ZVS occurs is revealed by examining the states in detail.
Each full cycle of the transformer has two distinct periods
in which power is delivered to the output, and two “free-
wheeling” periods. The two sides of the external bridge
have fundamentally different operating characteristics that
become important when designing for ZVS over a wide
load current range. The left bridge leg is referred to as the
“passive” leg, while the right leg is referred to as the
“active” leg. The following descriptions provide insight as
to why these differences exist.
State 1 (Power Pulse 1)
Referring to Figure 1, State 1 begins with MA, MD and MF
“ON” and MB, MC and ME “OFF.” During the simultaneous
conduction of MA and MD, the full input voltage is applied
across the transformer primary winding and following the
dot convention, V
allowing current to increase in LO1. The primary current
during this period is approximately equal to the output
inductor current (LO1) divided by the transformer turns
ratio plus the transformer magnetizing current (V
L
State 2 (Active Transition and Freewheel Interval)
MD turns off when the phase modulator comparator
transitions. At this instant, the voltage on the MD/MC
junction begins to rise towards the applied input voltage
(V
reflected output inductor current propels this action. The
slew rate is limited by MOSFET MC and MD’s output
capacitance (C
former interwinding capacitance. The voltage transition
MAG
IN
). The transformer’s magnetizing current and the
). MD turns off and ME turns on at the end of State 1.
LTC3722-1/LTC3722-2
OSS
IN
), snubbing capacitance and the trans-
/N is applied to the left side of LO1
IN
11
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372212i
ON
/