Detailed Description
The MAX5068 is a current-mode PWM controller for use
in isolated and nonisolated power-supply applications.
A bootstrap UVLO with a programmable hysteresis,
very low startup, and low operating current result in
high-efficiency universal-input power supplies. In addi-
tion to the internal bootstrap UVLO, the device also
offers programmable input startup and turn-off volt-
ages, programmed through the UVLO/EN input. When
using the MAX5068 in the bootstrapped mode, if the
power-supply output is shorted, the tertiary winding
voltage drops below the 10V threshold, causing the
bootstrap UVLO to turn off the gate drive to the external
power MOSFET, reinitiating a startup sequence with
soft-start.
The MAX5068 includes a cycle-by-cycle current limit
that turns off the gate drive to the external MOSFET
during an overcurrent condition. The MAX5068 integrat-
ing fault protection reduces average power dissipation
during persistent fault conditions (see the Integrating
Fault Protection section).
The MAX5068 features a very accurate, wide-range,
programmable oscillator that simplifies and optimizes
the design of the magnetics. The MAX5068A/C/D are
well suited for universal-input (rectified 85V
AC
to
265V
AC
) or telecom (-36V
DC
to -72V
DC
) power sup-
plies. The MAX5068B/E/F are well suited for low-input
voltage (10.8V
DC
to 24V
DC
) power supplies.
The MAX5068 high-frequency, universal input, offline/
telecom, current-mode PWM controller integrates all the
building blocks necessary for implementing AC-DC and
DC-DC fixed-frequency power supplies. Isolated or non-
isolated power supplies are easily constructed using
either primary- or secondary-side regulation. Current-
mode control with leading-edge blanking simplifies con-
trol-loop design, and an external slope-compensation
control stabilizes the current loop when operating at
duty cycles above 50% (MAX5068C/D/E/F). The
MAX5068A/B limit the maximum duty cycle to 50% for
use in single-ended forward converters. The
MAX5068C/D/E/F allow duty cycles up to 75% for use in
flyback converters.
An input undervoltage lockout (UVLO) programs the
input-supply startup voltage and ensures proper opera-
tion during brownout conditions. An external voltage-
divider programs the supply startup voltage. The
MAX5068A/B/D/F feature a programmable UVLO hys-
teresis. The MAX5068A/C/D feature an additional internal
bootstrap UVLO with large hysteresis that requires a min-
imum startup voltage of 23.6V. The MAX5068B/E/F start
up from a minimum voltage of 10.8V. Internal digital soft-
start reduces output-voltage overshoot at startup.
A single external resistor programs the switching fre-
quency from 12.5kHz to 1.25MHz. The MAX5068A/B/C/E
provide a SYNC input for synchronization to an external
clock. The maximum FET driver duty cycle is 50% for the
MAX5068A/B, and 75% for the MAX5068C/D/E/F.
Integrating fault protection ignores transient overcurrent
conditions for a set length of time. The length of time is
programmed by an external capacitor. The internal ther-
mal-shutdown circuit protects the device if the junction
temperature should exceed +170°C.
Power supplies designed with the MAX5068 use a
high-value startup resistor, R1, which charges a reser-
voir capacitor, C1 (Figure 1). During this initial period,
while the voltage is less than the internal bootstrap
UVLO threshold, the device typically consumes only
47µA of quiescent current. This low startup current and
the large bootstrap UVLO hysteresis help to minimize
the power dissipation across R1, even at the high end
of the universal AC input voltage (265V
AC
).
The MAX5068 includes a cycle-by-cycle current limit
that turns off the gate to the external MOSFET during an
overcurrent condition. When using the MAX5068A/C/D
in the bootstrap mode (if the power-supply output is
shorted), the tertiary winding voltage drops below the
9.74V bootstrap UVLO to turn off the gate to the exter-
nal power MOSFET. This reinitiates a startup sequence
with soft-start.
Current-Mode Control
The MAX5068 offers a current-mode control operation
feature, such as leading-edge blanking with a dual
internal path that only blanks the sensed current signal
applied to the input of the PWM controller. The current-
limit comparator monitors CS at all times and provides
cycle-by-cycle current limit without being blanked. The
leading-edge blanking of the CS signal prevents the
PWM comparator from prematurely terminating the on
cycle. The CS signal contains a leading-edge spike
that results from the MOSFET gate charge current, and
the capacitive and diode reverse-recovery current of
the power circuit. Since this leading-edge spike is nor-
mally lower than the current-limit comparator threshold,
current limiting is provided under all conditions.
Use the MAX5068C/D/E/F in flyback applications where
wide line voltage and load-current variations are
expected. Use the MAX5068A/B for forward/flyback
converters where the maximum duty must be limited to
less than 50%.
MAX5068
High-Frequency, Current-Mode PWM Controller
with Accurate Programmable Oscillator
_______________________________________________________________________________________ 9
