LT1952 Linear Technology, LT1952 Datasheet - Page 10

• Manufacturer Part Number: LT1952
• Description: Single Switch Synchronous Forward Controller
• Manufacturer: Linear Technology

OPERATIO
LT1952
Introduction
The LT1952 is a current mode synchronous PWM control-
ler optimized for control of the simplest forward converter
topology — using only one primary MOSFET. The LT1952
is ideal for 25W to 500W power systems where very high
efficiency and reliability, low complexity and cost are
required in a small space. Key features of the LT1952
include an adaptive maximum duty cycle clamp for the
single primary MOSFET. An additional output signal is
included for synchronous rectifier control. A precision
100mV threshold senses over-current conditions and
triggers Soft-Start for low stress short circuit protection
and control. The key functions of the LT1952 are shown in
the Block Diagram in Figure 2.
Part Startup
In normal operation the SD_V
and the V
turn on. This combination of pin voltages allows the 2.5V
V
circuitry and providing up to 2.5mA external drive. SD_V
threshold can be used for externally programming an
undervoltage lockout (UVLO) threshold on the system
input voltage. Hysteresis on the UVLO threshold can also
be programmed since the SD_V
before part turn on and 0µA after part turn on.
With the LT1952 turned on, the V
8.75V before part shutdown occurs. This V
esis (5.5V) combined with low 460µA start-up input
current allows low power start-up using a resistor/capaci-
tor network from system V
3). The V
below 8.75V before an auxiliary winding in the converter
takes over supply to the V
Output Drivers
The LT1952 has two outputs, SOUT and OUT. The OUT pin
provides a ±1A peak MOSFET gate drive clamped to 13V.
The SOUT pin has a ±50mA peak drive clamped to 12V and
provides sync signal timing for synchronous rectification
control.
10
REF
pin to become active, supplying the LT1952 control
IN
IN
capacitor value is chosen to prevent V
pin must exceed 14.25V to allow the part to
U
IN
IN
pin.
to supply the V
SEC
SEC
IN
pin must exceed 1.32V
pin can drop as low as
pin draws 11µA just
IN
IN
pin hyster-
pin (Figure
IN
falling
SEC
For SOUT and OUT turn on, a PWM latch is set at the start
of each main oscillator cycle. OUT turn on is delayed from
SOUT turn on by a time t
programmed using a resistor from the DELAY pin to
ground and is used to set the timing control of the
secondary synchronous rectifiers for optimum efficiency.
SOUT and OUT turn off at the same time each cycle by one
of three methods:
(1) MOSFET peak current sense at I
(2) Adaptive maximum duty cycle clamp reached during
(3) Maximum duty cycle reset of the PWM latch
During any of the following conditions — low V
SD_V
soft-start event is latched and both SOUT and OUT turn off
immediately (Figure 1).
Leading Edge Blanking
To prevent MOSFET switching noise causing premature
turn off of SOUT or OUT, programmable leading edge
blanking exists. This means both the current sense com-
parator and over-current comparator outputs are ignored
during MOSFET turn on and for an extended period after
the OUT leading edge (Figure 6). The extended blanking
period is programmable by adjusting a resistor from the
BLANK pin to ground.
Adaptive Maximum Duty Cycle Clamp
(Volt-Second Clamp)
For forward converter applications using the simplest
topology of a single MOSFET on the primary, a maximum
switch duty cycle clamp which adapts to transformer input
voltage is necessary for reliable control of the MOSFET.
This volt-second clamp provides a safeguard for trans-
former reset that prevents transformer saturation. Instan-
taneous load changes can cause the converter loop to
demand maximum duty cycle. If the maximum duty cycle
of the switch is too great, the transformer reset voltage can
exceed the voltage rating of the primary-side MOSFET with
catastrophic damage. Many converters solve this problem
load/line transients
SEC
or over-current detection at the OC pin — a
DELAY
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(Figure 2). t
SENSE
pin
DELAY
IN
, low
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is