SP6134H Datasheet, PDF (11/15 Pages) Sipex Corporation – High Voltage, 600 KHz Synchronous PWM Controller

SP6134H
High Voltage, 600kHz Synchronous PWM Step Down
Controller
copper pad directly under the package,
without occupying additional board space, can
increase the maximum power from
approximately 1 to 1.2W. For DPAK package,
enlarging the tap mounting pad to 1 square
inches reduces the RΘJA from 96°C/W to
40°C/W.
SCHOTTKY DIODE SELECTION
When paralleled with the bottom MOSFET, an
optional Schottky diode can improve efficiency
and reduce noises. Without this Schottky
diode, the body diode of the bottom MOSFET
conducts the current during the non-overlap
time when both MOSFETs are turned off.
Unfortunately, the body diode has high
forward voltage and reverse recovery problem.
The reverse recovery of the body diode causes
additional switching noises when the diode
turns off. The Schottky diode alleviates these
noises and additionally improves efficiency
thanks to its low forward voltage. The reverse
voltage across the diode is equal to input
voltage, and the diode must be able to handle
the peak current equal to the maximum load
current.
The power dissipation of the Schottky diode is
determined by
P_Diode = 2V_F I_OUTT_NOL F_S
where
TNOL = non-overlap time between GH and GL.
VF = forward voltage of the Schottky diode.
LOOP COMPENSATION DESIGN
The open loop gain of the whole system can
be divided into the gain of the error amplifier,
PWM modulator, buck converter output stage,
and feedback resistor divider. In order to
crossover at the selected frequency FCO, the
gain of the error amplifier has to compensate
for the attenuation caused by the rest of the
loop at this frequency. The goal of loop
compensation is to manipulate loop frequency
response such that its gain crosses over 0db
at a slope of -20db/dec. The first step of
compensation design is to pick the loop
crossover frequency. High crossover frequency
is desirable for fast transient response, but
often jeopardizes the system stability.
Crossover frequency should be higher than the
ESR zero but less than 1/5 of the switching
frequency. The ESR zero is contributed by the
ESR associated with the output capacitors and
can be determined by:
f _{Z (ESR)}
=
1
2πC_OUT R_ESR
The next step is to calculated the complex
conjugate poles contributed by the LC output
filter,
f _{P(LC )}
=
2π
1
LC
When the output capacitors are of a Ceramic
Type, the SP6134HCU Evaluation Board
requires a Type III compensation circuit to
give a phase boost of 180° in order to
counteract the effects of an under damped
resonance of the output filter at the double
pole frequency.
© 2008 Exar Corporation
11/15
Rev. 2.0.0