Datasheet

MAX17598/MAX17599
Low I
Q
, Wide-Input Range, Active Clamp
Current-Mode PWM Controllers
18Maxim Integrated
n-Channel MOSFET Gate Driver
The NDRV output drives an external n-channel MOSFET.
NDRV can source/sink in excess of 900mA /1500mA
peak current. Therefore, select a MOSFET that yields
acceptable conduction and switching losses.
p-Channel MOSFET Gate Driver
The AUXDRV output drives an external p-channel
MOSFET with the aid of a level shifter, as shown in the
Typical Application Circuits. AUXDRV can source/sink in
excess of 300mA/700mA peak current. Therefore, select
a MOSFET that yields acceptable conduction and switch-
ing losses. The external PMOSFET used must be able to
withstand the maximum clamp voltage.
Dead Time
Dead time between the main and AUX output edges
allow ZVS to occur, minimizing switching losses and
improving efficiency. The dead time (t
DT
) is applied to
both leading and trailing edges of the main and AUX
outputs as shown in Figure 7. Connect a resistor between
DT and GND to set t
DT
to any value between 25ns and
250ns. RDT in kΩ, is calculated as:
=×W
DT DT
R 0.4 t k ,
where t
DT
is in ns.
Oscillator/Switching Frequency
The ICs’ switching frequency is programmable between
100kHz and 1MHz with a resistor R
RT
connected between
RT and GND. Use the following formula to determine the
appropriate value of R
RT
needed to generate the desired
output switching frequency (f
SW
):
10
RT
SW
1 10
R
f
×
=
where f
SW
is the desired switching frequency.
Peak-Current-Limit
The current-sense resistor (R
CS
), connected between
the source of the n-channel MOSFET and PGND, sets
the current limit. The source end of current-sense
resistor connects to CS pin of MAX17598/MAX17599.
The signal thus obtained is used by the devices,
both for current-mode control and peak-current limiting
purposes. The current-limit comparator has a voltage trip
level (V
CS-PEAK
) of 305mV, and is independent of slope
compensation applied to stabilize the converter. The
following equation is used to calculate the value of R
CS
:
=
×
CS
PRI_PEAK
305mV
R
1.2 I
where I
PRI_PEAK
is the peak current in the primary side
of the transformer, which also flows through the main
n-channel MOSFET. When the voltage produced by this
current (through the current-sense resistor) exceeds the
current-limit comparator threshold, the MOSFET driver
(NDRV) terminates the current on-cycle within 40ns (typ).
The devices implement 70ns of internal leading-edge
blanking to ignore leading-edge current spikes encoun-
tered in practice due to parasitics. Use a small RC
network for additional filtering of the leading-edge spike
on the sense waveform when needed. Set the corner
frequency of the RC filter network at 5 to 10 times the
switching frequency.
For a given peak-current-limit setting, the runaway cur-
rent limit is typically 20% higher. The peak current-limit-
triggered hiccup operation is disabled until the end of
soft-start, while the runaway current-limit-triggered hiccup
operation is always enabled.
Negative Peak Current Limit
The MAX17598/MAX17599 protect against excessive
negative currents through the clamp switch, primary of
the transformer and the clamp capacitor under dynamic
operating conditions. The devices limit negative current
by monitoring the voltage across R
CS
, while the AUXDRV
output is low and the p-Channel FET is on. The typical
negative-current-limit threshold is set at -102mV (1/3 of
the positive-peak-current-limit threshold).
Figure 7. Dead Time Between AUXDRV and NDRV
AUXDRV
NDRV
DEAD TIME, t
DT