Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
17Maxim Integrated
Programming the Output Voltage of
the Flyback/Boost Converter (EAFN)
Set the output voltage of the flyback/boost converter by
selecting the correct values for the resistor-divider con-
nected from the flyback/boost output to ground (V
OUTF
)
with the midpoint of the divider connected to the EAFN
pin (Figure 8). With R
B
selected in the range of 20kI to
50kI, R
U
can be calculated as:
OUTF
UB
V
R R 1k
1.22

= × −Ω


where R
B
is in kI.
Programming the Current Limit of the
Flyback/Boost Converter (RLIMF)
The devices include a robust overcurrent-protection
scheme that protects them during overload and short-
circuit conditions. For the flyback/boost converter, the
devices include a cycle-by-cycle peak current limit that
turns off the driver whenever the current into the LXF pin
exceeds an internal limit programmed by the resistor
connected from the RLIMF pin to ground. The devices
include a runaway current limit that protects them during
short-circuit conditions. One occurrence of the runaway
current limit trigger a hiccup mode, protecting the con-
verter by immediately suspending switching for a period
of time (32ms). This allows the overload current to decay
due to power loss in the converter resistances, load, and
the output diode of the flyback/boost converter before
soft-start is attempted again. The R
LIMF
resistor for a
desired current limit (I
PK
) can be calculated as:
LIMF PK
R 50 I k=×Ω
where I
PK
is expressed in amperes.
For a given peak-current-limit setting, the runaway
current limit is typically 20% higher. The runaway current-
limit-triggered hiccup operation is always enabled, even
during soft-start operation.
Programming the Slope Compensation for
the Flyback/Boost Converter (SCOMPF)
When both devices operate at maximum duty cycle of
49%, in theory they do not require slope compensation
to prevent subharmonic instability that occurs naturally
in continuous peak-current-mode controlled converters
operating at duty cycles greater than 50%. In practice,
the MAX17497A requires a minimum amount of slope
compensation to provide stable, jitter-free operation. The
MAX17497A allows the user to program this default value
of slope compensation simply by connecting the RLIMF
pin to V
CC
. It is recommended that discontinuous-mode
designs also use this minimum amount of slope compen-
sation to provide noise immunity and jitter-free operation.
Figure 7. MAX17497B Typical Startup Circuit with Bias Winding
to Turn Off Q1 and Reduce Power Dissipation
Figure 8. Programming the Output Voltage of the Flyback/Boost
Converter
V
DC
V
OUTF
LXF
Np Ns
D2
D1
IN
C
OUTF
MAX17497B
IN
NB
V
CC
C
VCC
C
IN
R
Z
Z
D1
6.3V
LDO
Q1
R
B
R
U
EAFN
V
OUTF
MAX17497A
MAX17497B