Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
18Maxim Integrated
The MAX17497A flyback/boost converter can be
designed to operate in discontinuous mode or to enter
into continuous-conduction mode at a specific heavy-
load condition for a given DC input voltage. In continu-
ous-conduction mode, the flyback/boost converter needs
slope compensation to avoid subharmonic instability that
occurs naturally over all specified load and line condi-
tions in peak-current-mode-controlled converters operat-
ing 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 cycles
to provide stable, jitter-free operation. The SCOMPF pin
allows the user to program the necessary slope com-
pensation by setting the value of the R
SCOMPF
resistor
connected from the SCOMPF pin to ground:
SCOMPF E
R 0.5 S k= Ω
where the slope (S
E
) is expressed in millivolts per micro-
second.
Step-Down Overcurrent Protection
The devices’ step-down regulator includes a robust
overcurrent-protection scheme that protects them dur-
ing overload and short-circuit conditions. A runaway
current limit on the high-side switch current at 1A (typ)
protects the device under short-circuit conditions. One
occurrence of the runaway current limit trigger a hiccup
mode to protect the converter by immediately suspend-
ing switching for 32ms. This allows the overload current
to decay, due to power loss in the converter resistances,
and load before soft-start is attempted again.
Error Amplifier, Loop Compensation,
and Power-Stage Design of the
Flyback/Boost Converter
The devices’ flyback/boost converter requires that prop-
er loop compensation be applied to the error-amplifier
output to achieve stable operation. The goal of the com-
pensator design is to achieve the desired closed-loop
bandwidth and sufficient phase margin at the crossover
frequency of the open-loop gain-transfer function of the
converter. The error amplifier included in the devices is a
transconductance amplifier. The compensation network
used to apply the necessary loop compensation is shown
in Figure 9.
The flyback/boost converter can be used to implement
the following converters and operating modes:
• Nonisolated flyback converter in discontinuous-
conduction mode (DCM flyback)
• Nonisolatedflybackconverterincontinuous-conduc-
tion mode (CCM flyback)
• Boost converter in discontinuous-conduction mode
(DCM boost)
• Boost converter in continuous-conduction mode
(CCM boost)
Calculations for loop-compensation values (R
Z
, C
Z
, and
C
P
) for these converter types, and design procedures for
power-stage components, are detailed in the following
sections.
DCM Flyback
Primary Inductance Selection
In a DCM flyback converter, the energy stored in the
primary inductance of the flyback transformer is ideally
delivered entirely to the output. The maximum primary-
inductance value for which the converter remains in
discontinuous mode at all operating conditions can be
calculated as:
( )
( )
2
INMIN MAX
PRIMAX
OUTF D OUTF SW
V D 0.4
L
V VI f
××
≤
+× ×
where D
MAX
is 0.35 for the MAX17497A and 0.7 for the
MAX17497B, V
D
is the forward-voltage drop of the out-
put rectifier diode on the secondary side, and f
SW
is the
switching frequency of the power converter. Choose the
primary inductance value to be less than L
PRIMAX
.
Figure 9. Programming the Output Voltage of the Flyback/Boost
Converter
R
Z
COMPF
C
Z
C
P
MAX17497A
MAX17497B