Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
15Maxim Integrated
Figure 3. Programming EN/UVLO and OVI
Figure 4. MAX17497A RC-Based Startup Circuit
Startup Operation
The MAX17497A is optimized for implementing offline
flyback converters. A cost-effective RC startup circuit
is used in offline applications. In this startup method,
when the input DC voltage is applied, the startup
resistor (R
START
) charges the startup capacitor (C
START
),
causing the voltage at the IN pin to increase towards the
rising IN UVLO threshold (20V typ). During this time, the
MAX17497A draws a low startup current of 20FA (typ)
through R
START
. When the voltage at IN reaches the
rising IN UVLO threshold, the MAX17497A commences
switching operations and drives the internal nMOSFET
whose drain is connected to the LXF pin. In this con-
dition, the MAX17497A draws 2.5mA current in from
C
START
, in addition to the current required to switch the
gate of the external nMOSFET (Q1). Since this current
cannot be supported by the current through R
START
,
the voltage on C
START
starts to drop. When suitably
configured as show in Figure 4, the external nMOSFET
is switched by the LXF pin and the flyback converter
generates an output voltage (V
OUTF
) bootstrapped to
the IN pin through the diode (D2). If V
OUTF
exceeds
the sum of 6V and the drop across D2 before the
voltage on C
START
falls below 5V, then the IN volt-
age is sustained by V
OUTF
, allowing the MAX17497A
to continue operating with energy from V
OUTF
. The
large hysteresis (15V typ) of the MAX17497A allows
for a small startup capacitor (C
START
). The low startup
current (20FA typ) allows the use of a large startup resis-
tor (R
START
), thus reducing power dissipation at higher
DC bus voltages. R
START
may need to be implemented
as equal, multiple resistors in series (R
IN1
, R
IN2
and
R
IN3
) to share the applied high DC voltage in offline
applications, such that the voltage across each resistor
is limited to the maximum continuous operating-voltage
rating. R
START
and C
START
can be calculated as:
GATE sw SSF
START IN
6
Q ft
C I µF
10
10


×
=+×




where I
IN
is the supply current drawn at the IN pin in mA,
Q
GATE
is the gate charge of the external MOSFET used in
nC, f
SW
is the switching frequency of the converter in Hz,
and t
SSF
is the soft-start time programmed for the flyback
converter in ms (see the Programming the Soft-Start of the
Flyback/Boost Converter (SSF) section).
( )
START
START
START
V 10 50
Rk
1C
−×
= Ω
+


where C
START
is the startup capacitor in FF.
For designs that cannot accept power dissipation in
the startup resistors at high DC input voltages in offline
applications, the startup circuit can be set up with a
current source instead of a startup resistor, as shown
in Figure 5.
OVI
R
OVI
R
EN
R
SUM
R
DC3
R
DC2
R
DC1
V
DC
EN/UVLO
MAX17497A
MAX17497B
R
START
R
IN3
V
OUTF
D2
V
DC
IN
LXF
V
CC
C
VCC
C
START
R
IN2
R
IN1
MAX17497A
LDO
V
DC
V
OUTF
D1
C
OUTF