Datasheet

MAX17598/MAX17599
Low I
Q
, Wide-Input Range, Active Clamp
Current-Mode PWM Controllers
15Maxim Integrated
Startup Operation
The MAX17598 is optimized for implementing active-
clamp converters operating either from a rectified AC
input or in a 36V DC to 72V
DC
telecom application.
A cost-effective RC startup circuit can be used in such
applications. In this startup method (Figure 3), when
the input DC voltage is applied, the startup resistor
R
START
charges the startup capacitor C
START
, causing
the voltage at the V
IN
pin to increase towards the rising
V
IN
UVLO threshold (20V typical). During this time, the
MAX17598 draws a low startup current of 20FA (typ)
through the startup resistor R
START
. When the voltage at
V
IN
reaches the rising IN UVLO threshold, the MAX17598
commences all internal operations and drives the exter-
nal MOSFETs connected to NDRV and AUXDRV. In this
condition, the MAX17598 draws 2mA (typ) current in
from C
START
, in addition to the current required to switch
the gates of the external MOSFETs Q1and Q2. Since
this current cannot be supported by the current through
R
START
, the voltage on C
START
starts to drop. When
suitably configured as shown in Figure 3, the converter
operates to generate an output voltage (V
BIAS
) that is
bootstrapped to the V
IN
pin. If the voltage V
BIAS
exceeds
7V before the voltage on C
START
falls below 7V (typ),
then the V
IN
voltage is sustained by V
BIAS
, thus allow-
ing the MAX17598 to continue to operate with energy
from V
BIAS
. The large hysteresis (13V typical) of the
MAX17598 allows for a small startup capacitor (C
START
).
The low startup current (20FA typical) allows the use of
a large startup resistor (R
START
), thus reducing power
dissipation at higher DC bus voltages. The startup resis-
tor R
START
might need to be implemented as equal, mul-
tiple resistors in series (R
IN1
, R
IN2
and R
IN3
) to share the
applied high DC voltage in offline applications so that the
voltage across each resistor is limited to the maximum
continuous operating voltage rating. R
START
and C
START
can be calculated as follows:
× + ××


=

×

++




VDRV IN SS
START
GATE sw
IN SS
6
7.4 C 0.04 I C
C 0.09 µF
Qf
It
10
where I
IN
is the supply current drawn at the IN pin in
mA, Q
GATE
is the sum of the gate charges of the exter-
nal MOSFETs Q1 and Q2 in nC, f
sw
is the switching
frequency of the converter in Hz, t
SS
is the soft-start
time programmed for the converter in ms. C
VDRV
is
cummulative capacitor used at DRV node in μF, and
C
SS
is soft-start capacitor in nF. See the Soft-Start
section.
( )
START
START
START
V 10 50
R k,
1C
−×
= W
+


where C
START
is the startup capacitor in FF.
The IN UVLO rising threshold of the MAX17599 is set
to 4.1V with a hysteresis of 200mV, and is optimized for
low-voltage DC-DC applications in the range of 4.5V DC
to 36V DC. The IN pin is rated for a maximum operating
input voltage of 36V DC and can directly be connected
to the input DC supply.
Figure 3. RC-Based Startup Circuit
R
IN1
R
IN2
R
IN3
LDO
V
IN
V
DC
V
BIAS
D1
D2
L
BIAS
C
CLAMP
V
DC
AUXDRV
NDRV
Q1
Q2
V
DRV
C
VDRV
C
START
R
START
MAX17598