lt3804 Linear Technology Corporation, lt3804 Datasheet - Page 11

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

APPLICATIO S I FOR ATIO
Light Load Operation of Second Output
If the BGS pin is grounded, the LT3804 stays in continuous
mode independent of load condition except during soft-
start operation (see the Soft-Start section). If the BGS pin
is left open under light load, V
BGATE will be turned off(see comparator CA2 of Block
Diagram), and the LT3804 will enter discontinous mode
operation.
Second Output Capacitor Selection
The selection of the output capacitor is determined by the
output ripple and load transient requirements. In low
output voltage applications, always choose capacitors
with low ESR. The output ripple voltage is approximated
by:
where I
partial list of low ESR high performance capacitor types
includes SP capacitors from Panasonic and Cornell Dubilier,
POSCAPs and OS-CON capacitors from Sanyo, T510 and
T520 surface mount capacitors from Kemet.
Layout Considerations
For maximum efficiency, the switching rise and fall times
should be less than 20ns. To prevent radiation, the power
MOSFETs, SW pin and input bypass capacitor leads should
be kept as short as possible. A ground plane should be
used under the switching circuitry to prevent interplane
coupling and to act as a thermal spreading path. Note that
the bottom metal of the package is the heat sink as well as
the IC signal ground, and must be soldered to the ground
plane.
V
OUT
L
is the inductor peak-to-peak ripple current. A
I ESR
L
U
8
U
fC
1
OUT
RS2
will drop below 8mV,
W
U
Design Example
Figure 4 shows an application example of LT3804. It is a
dual output high efficiency isolated DC/DC power supply
with 36V to 72V input range, 3.3V/15A and 1.8V/15A
outputs. The basic power stage topology is a two-switch
forward converter with synchronous rectification. The
primary side controller uses an LT3781, a current mode
two-switch forward controller with built-in MOSFET driv-
ers. On the secondary side, the LT3804 is used to provide
the voltage feedback for the 3.3V output. The output from
the built-in optocoupler driver is fed into an optocoupler
(MOC207) and then transferred to LT3781 on the primary
side to complete the 3.3V regulation. An LTC1693-1 high
speed dual N-channel MOSFET driver provides the gate
drive for the synchronous MOSFETs at the 3.3V output
stage. The LTC1693-1 driver’s input signals come from
SG and BG outputs of the LT3781 through two small gate
drive transformers (T2 and T3).
The LT3804 also precisely regulates the 1.8V output by
further reducing and controlling the duty cycle of the
switching waveform from the power transformer (T1)
secondary winding. In fact, the 1.8V circuit is a special
synchronous buck converter whose input is a pulsed
waveform instead of a DC voltage.
True differential remote sensing is provided for both
outputs to achieve high regulation accuracies. Power
good indicator PGOOD will be high only if both outputs are
within 10% of their nominal values.
The LT3804 provides current limit function for both 1.8V
and 3.3V outputs. The current limits for 3.3V and 1.8V
outputs are estimated to be 50mV/R55 and 50mV/R49,
respectively.
LT3804
11
3804i