lt3804 Linear Technology Corporation, lt3804 Datasheet - Page 7

• Manufacturer Part Number: lt3804
• Description: Secondary Side Dual Output Controller With Opto Driver
• Manufacturer: Linear Technology Corporation

OPERATIO
To generate isolated multiple outputs, most systems use
either multiple secondary windings or cascade regulators
for each additional output. Multiple secondary windings
sacrifice regulation of the auxiliary outputs. Cascaded
regulators require a larger inductor for the main output,
because all of the power is processed in series. By gener-
ating the auxiliary output(s) from the secondary winding
of the main output, the LT3804 allows for parallel process-
ing of the output power. This minimizes the main output
inductor size and directly regulates the auxiliary output.
With synchronous rectification, the system efficiency is
greatly improved.
The LT3804 regulates both the main and one auxiliary
output, with true remote sensing to achieve high output
accuracy. To regulate the first output, the LT3804 contains
a high gain error amplifier VA1 and an optocoupler driver
OA1 with a unique feature that reduces output overshoot
to a minimum. For details see the Applications Information
section. The second output includes a voltage amplifier,
VA2, (see Block Diagram)a voltage mode PWM with
trailing edge synchronization and leading edge modula-
tion, a current limit amplifier, CA2, and high speed syn-
chronous switch drivers.
U
Figure 2. Leading Edge Modulation, Trailing Edge Synchronization
SECONDARY VOLTAGE
SYNC SIGNAL V
SWITCH NODE V
TRANSFORMER
RAMP V
V
BGATE
TGATE
AOUT2
RESET
CSET
I
SW
L2
V
S
V
RESET
D
1
T
T
During normal operation (see Figure 2), a switching cycle
begins at the falling edge of the transformer secondary
voltage V
The internal oscillator is reset, turning off the top MOSFET,
M1, and turning on the bottom MOSFET, M2. During this
portion of the cycle, the inductor current is discharged by
the output voltage V
voltage, V
Since M1 is off, the switch node voltage, V
zero. The inductor current continues to be discharged by
the output voltage V
ramp signal intersects the feedback error amplifier output
V
node voltage to V
circuit is then charged by V
of this buck circuit ends when the secondary voltage
becomes zero. The next cycle repeats. The ideal equation
for duty cycle of the LT3804 is:
where V
amplitude of the secondary voltage and D2 is the duty
cycle of the switching node voltage V
Figure 2.
T
D
AOUT2
2
T
D2 = V
. The top MOSFET M1 turns on, pulling the switch
OUT2
S
OUT2
S
.
, will go high during this portion of the cycle.
is the auxiliary output voltage, V
/V
SP
S
3710 F02
. The inductor current of the LT3804
V
V
SP
SP
OUT2
OUT2
. This condition lasts until the
S
. The transformer secondary
– V
OUT2
. The effective on time
SW
, as defined in
LT3804
SW
, remains
SP
is the
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3804i