Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
20Maxim Integrated
is on, and the output-voltage ripple is therefore a function
of load current and duty cycle. Use the following equa-
tion to calculate the output-capacitor ripple:
( )
−

××

∆=
× ××
2
OUT PRIPEAK OUTF
COUTF
2
PRIMPEAK SW OUTF
I I KI
V
2I f C
where I
OUTF
is the load current and D
NEW
is the duty
cycle at minimum input voltage.
Input-Capacitor Selection
The MAX17497A is optimized for implementing offline
AC-DC converters. In such applications, the input capac-
itor must be selected based either on the ripple due to
the rectified line voltage or on hold-up time requirements.
Hold-up time can be defined as the time period over
which the power supply should regulate its output volt-
age from the instant the AC power fails. The MAX17497B
is useful for implementing low-voltage DC-DC applica-
tions where the switching-frequency ripple must be
used to calculate the input capacitor. In both cases, the
capacitor must be sized to meet RMS current require-
ments for reliable operation.
Capacitor Selection Based on Switching Ripple
(MAX17497B): For DC-DC applications, X7R ceramic
capacitors are recommended due to their stability over
the operating temperature range. The ESR and ESL of a
ceramic capacitor are relatively low, so the ripple voltage
is dominated by the capacitive component. For the fly-
back converter, the input capacitor supplies the current
when the main switch is on. The following equation cal-
culates the input capacitor for a specified peak-to-peak
input switching-ripple voltage (V
IN_RIP
):
( )
2
NEW PRIPEAK NEW
IN
SW IN_RIP
D I 1 0.5 D
C
2f V

× −×

=
××
Capacitor Selection Based on Rectified Line Voltage
Ripple (MAX17497A): For the flyback converter, the
input capacitor supplies the input current when the diode
rectifier is off:
×
=
η×
LOAD
IN
2
INPK
0.45 P
C
V
where P
LOAD
is the rated output power, V
INPK
is the
peak voltage at minimum input, and η is the efficiency at
minimum input at maximum load.
Capacitor Selection Based on Hold-Up Time
Requirements (MAX17497A): For a given output power
(P
HOLDUP
) that needs to be delivered during hold-up
time (t
HOLDUP
), DC bus voltage at which the AC supply
fails (V
INFAIL
), and the minimum DC bus voltage at which
the converter can regulate the output voltages (V
INMIN
),
the input capacitor (C
IN
) is estimated as:
(
)
HOLDUP HOLDUP
IN
22
INFAIL INMIN
3P t
C
VV
××
=
−
The input capacitor RMS current can be calculated as
follows:
×
=
η×
LOAD
INCRMS
INPK
2.7 P
I
V
External MOSFET Selection
MOSFET selection criteria includes the maximum drain
voltage, peak/RMS current in the primary, and the
maximum allowable power dissipation of the package
without exceeding the junction temperature limits. The
voltage seen by the MOSFET drain is the sum of the input
voltage, the reflected secondary voltage on the trans-
former primary, and the leakage inductance spike. The
MOSFET’s absolute maximum V
DS
rating must be higher
than the worst-case drain voltage:
OUTF D
DSMAX INMAX
VV
V V 2.5
K

+

=+×




The drain-current rating of the external MOSFET is
selected to be greater than the worst-case peak current-
limit setting.
Secondary Diode Selection
Secondary diode-selection criteria includes the maxi-
mum reverse voltage, average current in the secondary,
reverse recovery time, junction capacitance, and the
maximum allowable power dissipation of the package.
The voltage stress on the diode is the sum of the output
voltage and the reflected primary voltage.
The maximum operating reverse-voltage rating must be
higher than the worst-case reverse voltage:
SECDIODE INMAX OUTF
V 1.25 (K V V )= ×× +
The current rating of the secondary diode should be
selected such that the power loss in the diode (given
as the product of forward-voltage drop and the average