7
LT1425
TI I G DIAGRA
W
W
U
V
SW
VOLTAGE
V
IN
GND
OFF
ON
MINIMUM t
ON
ENABLE DELAY
MINIMUM ENABLE TIME
1425 TD
OFF
ON
SWITCH
STATE
FLYBACK AMP
STATE
0.80
×
V
FLBK
V
FLBK
COLLAPSE
DETECT
ENABLED
DISABLED
DISABLED
OPERATIO
N
U
The LT1425 is a current mode switching regulator IC that
has been designed specifically for the isolated flyback
topology. The special problem normally encountered in
such circuits is that information relating to the output
voltage on the isolated secondary side of the transformer
must be communicated to the primary side in order to
maintain regulation. Historically, this has been done with
optoisolators or extra transformer windings. Optoisolator
circuits waste output power and the extra components
they require increase the cost and physical volume of the
power supply. Optoisolators can also exhibit trouble due
to limited dynamic response (temporal), nonlinearity,
unit-to-unit variation and aging over life. Circuits
employing extra transformer windings also exhibit defi-
ciencies. The extra winding adds to the transformer’s
physical size and cost. Dynamic response is often
mediocre. There is usually no method for maintaining
load regulation versus load.
The LT1425 derives its information about the isolated
output voltage by examining the primary side flyback
pulse waveform. In this manner no optoisolator nor extra
transformer winding is required. This IC is a quantum
improvement over previous approaches because: target
output voltage is directly resistor-programmable, regu-
lation is maintained well into discontinuous mode and
optional load compensation is available.
The Block Diagram shows an overall view of the system.
Many of the blocks are similar to those found in tradi-
tional designs including: internal bias regulator, oscilla-
tor, logic, current amplifier and comparator, driver and
output switch. The novel sections include a special
flyback error amplifier and a load compensation mecha-
nism. Also, due to the special dynamic requirements of
flyback control, the logic system contains additional
functionality not found in conventional designs.
