Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
25Maxim Integrated
value of the input inductor taking into account tolerance
and saturation effects. f
SWMIN
is the minimum switch-
ing frequency for the MAX17497B from the Electrical
Characteristics section.
Output-Capacitor Selection
X7R ceramic output capacitors are preferred in industrial
applications due to their stability over temperature. The
output capacitor is usually sized to support a step load
of 50% of the maximum output current in the application,
such that the output-voltage deviation is contained to 3%
of the output-voltage change. The output capacitance
can be calculated as:
STEP RESPONSE
OUTF
OUTF
It
C
V
×
=
∆
RESPONSE
C SW
0.33 1
t ()
ff
≅+
where I
STEP
is the load step, t
RESPONSE
is the response
time of the controller, DV
OUTF
is the allowable output-
voltage deviation, and f
C
is the target closed-loop
crossover frequency. f
C
is chosen as 1/10 the switch-
ing frequency (f
SW
). For the boost converter, the output
capacitor supplies the load current when the main switch
is on, and therefore the output-voltage ripple is a function
of duty cycle and load current. Use the following equa-
tion to calculate the output-capacitor ripple:
OUTF MAX
COUTF
OUTF SW
ID
V
Cf
×
∆=
×
Input-Capacitor Selection
The input ceramic capacitor value required can be
calculated based on the ripple allowed on the input DC
bus. The input capacitor should be sized based on the
RMS value of the AC current handled by it. The calcula-
tions are as:
OUTF
INF
INMIN SW MAX
3.75 I
C
V f (1 D )

×
=

× ×−

The input-capacitor RMS current can be calculated as:
LIN
CIN_RMS
I
I
23
∆
=
×
where:

× ×−
∆= <

×

OUTF MAX MAX
LIN MAX
IN SW
V D (1 D )
I for D 0.5
Lf

×
∆= ≥

×


OUTF
LIN MAX
IN SW
0.25 V
I for D 0.5
Lf
Error-Amplifier Compensation Design
The loop-compensation values for the error amplifier can
now be calculated as:
× × ×−
=
×
2
OUTF OUTF MAX
Z
OUTF IN
90 V C (1 D )
R
IL
where D
MAX
is the duty cycle at the lowest operating input
voltage and I
OUTF_MAX
is the maximum load current:
OUTF OUTF
Z
OUTF Z
VC
C
2I R
×
=
××
P
SW Z
1
C
fR
=
π× ×
Slope-Compensation Ramp
The slope required to stabilize the converter at duty
cycles greater than 50% can be calculated as:
×−
=
OUTF INMIN
E
IN
0.41 (V V )
S V per µs
L
where L
IN
is in FH.
Output Diode Selection
The design procedure for output diode selection is identi-
cal to that outlined in the DCM Boost section.