Datasheet

MAX17598/MAX17599
Low I
Q
, Wide-Input Range, Active Clamp
Current-Mode PWM Controllers
20Maxim Integrated
A current source at SYNC/DITHER charges the capacitor
C
DITHER
to 2V with 50FA. Upon reaching this trip point,
it discharges C
DITHER
to 0.4V with 50FA. The charging
and discharging of the capacitor generates a triangular
waveform on SYNC/DITHER with peak levels at 0.4V and
2V. C
DITHER
is calculated as:
=
DITHER
TRI
15.625
C nF
f
where f
TRI
is in kHz.
Typically, f
TRI
should be set close to 1kHz. The resistor
R
DITHER
connected from SYNC/DITHER to RT deter-
mines the amount of dither as follows:
RT
DITHER
R
%DITHER
R
=
where %DITHER is the amount of dither expressed as a
percentage of the switching frequency. Setting R
DITHER
to 10 x R
RT
generates Q10% dither.
Layout Recommendations
All connections carrying pulsed currents must be very
short and as wide as possible. The inductance of these
connections must be kept to an absolute minimum due
to the high di/dt of the currents in high-frequency switch-
ing power converters. This implies that the loop areas for
forward- and return-pulsed currents in various parts of the
circuit should be minimized. Additionally, small current
loop areas reduce radiated EMI. Similarly, the heatsink of
the MOSFET presents a dV/dt source. Therefore, the sur-
face area of the MOSFET heatsink should be minimized as
much as possible.
Ground planes must be kept as intact as possible. The
ground plane for the power section of the converter should
be kept separate from the analog ground plane, except for
a connection at the least-noisy section of the power ground
plane, typically the return of the input filter capacitor. The
negative terminal of the filter capacitor, the ground return
of the power switch, and current-sensing resistor must
be close together. PCB layout also affects the thermal
performance of the design. A number of thermal vias that
connect to a large ground plane should be provided under
the exposed pad of the part for efficient heat dissipation.
For a sample layout that ensures first pass success, please
refer to the MAX17598/MAX17599 Evaluation Kit layouts
available at www.maximintegrated.com. For universal
AC input designs, follow all applicable safety regulations.
Offline power supplies can require UL, VDE, and other
similar agency approvals.