Datasheet

MAX17598/MAX17599
Low I
Q
, Wide-Input Range, Active Clamp
Current-Mode PWM Controllers
16Maxim Integrated
Soft-Start and Soft-Stop
In a current-mode isolated active clamp forward con-
verter, the COMP voltage programs the peak current
in the primary, and thus the secondary-side inductor
current as well. The MAX17598/MAX17599 implement a
soft-start scheme that controls the COMP pin of the
device at turn on. A useful benefit of this feature is the
elimination of need for secondary-side soft-start circuitry
in such isolated applications. In the absence of sec-
ondary-side soft-start circuitry, the secondary-side error
amplifier can cause the output voltage to rapidly reach
the regulation value, thus causing inrush current and
output voltage overshoot. The MAX17598/MAX17599
avoid this issue by applying a soft-start to the COMP
pin. Thus the regulator’s primary and secondary currents
are ramped up in a well-controlled manner resulting in a
current-mode soft-start operation.
Soft-start period of MAX17598/MAX17599 can be pro-
grammed by selecting the value of capacitor connected
from SS pin to GND. The capacitor C
SS
can be calcu-
lated as follows:
×
=
−
SS
SS
COMP
10 t
C nF
V 1.81
where t
SS
is in ms, V
COMP
is steady-state COMP voltage
(V
COMP,MAX
= 2.6V).
A soft-stop feature ramps down the output voltage when
the device is turned off, and provides safe discharging
of the clamp capacitor, thus allowing the controller to
restart in a well-controlled manner. Additionally, a nega-
tive current limit is provided in the current-sense circuitry
that helps limit the clamp switch current under dynamic
operating conditions, such as momentary input overvolt-
age charging from a precharged output capacitor. The
soft-stop duration is twice that of the programmed soft-
start period.
Programming Slope Compensation
Since the MAX17598/MAX17599 operate at a maximum
duty cycle of 72.5% (typ), slope compensation is required
to prevent subharmonic instability that occurs naturally
in continuous-conduction mode, peak current mode-
controlled converters operating at duty cycles greater
than 50%. A minimum amount of slope signal is added to
the sensed current signal, even for converters operating
below 50% duty to provide stable, jitter-free operation.
The SLOPE pin allows the user to program the necessary
slope compensation by setting the value of the resistor
R
SLOPE
connected from SLOPE pin to ground.
E
SLOPE
S8
Rk
1.55
−
= W
where S
E
, the slope is expressed in mV per microseconds.
For the default minimum slope compensation of 50mV/Fs
(typ), the SLOPE pin should be connected to SGND or
left unconnected.
Figure 4. Typical Startup Circuit with IN Connected Directly to DC Input
MAX17599
LDO
V
IN
4.5V TO 36V DC
V
OUT
D1
D2
L
OUT
C
CLAMP
AUXDRV
NDRV
Q1
V
DRV
C
VDRV
C
OUT
Q2