Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
23Maxim Integrated
DCM Boost
In a DCM boost converter, the inductor current returns to
zero in every switching cycle. Energy stored during the
on-time of the main switch is delivered entirely to the load
in each switching cycle.
Inductance Selection
The design procedure starts with calculating the boost
converter’s input inductor, such that it operates in DCM
at all operating line and load conditions. The critical
inductance required to maintain DCM operation is cal-
culated as:
( )
2
OUTF INMIN INMIN
IN
2
OUTF OUTF SW
V V V 0.4
L
IV f

−× ×


≤
××
where V
INMIN
is the minimum input voltage.
Peak/RMS Current Calculation
To set the current limit, the peak current in the inductor
can be calculated as:
LIMF PK
I I 1.2= ×
where I
PK
is given by:
OUTF INMIN OUTF
PK
INMIN SWMIN
2 (V V ) I
I
Lf

×−×
=

×

L
INMIN
is the minimum value of the input inductor, taking
into account tolerance and saturation effects. f
SWMIN
is
the minimum switching 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
OUT
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 cross-
over frequency. f
C
is chosen to be 1/10 the switching
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 IN PK
COUTF
INMIN OUTF
I LI
V
VC
××
∆=
×
Input-Capacitor Selection
The value of the required input capacitor can be calcu-
lated based on the ripple allowed on the input DC bus.
The size of the input capacitor should be based on the
RMS value of the AC current handled by it. The calcula-
tions are as:
OUTF
INF
INMIN SWMIN MAX
3.75 I
C
V f (1 D )

×
=

× ×−

The capacitor RMS can be calculated as:
PK
CIN_RMS
I
I
23
=
×
Error-Amplifier Compensation Design
The DC gain of the power stage is given as:
× − ×× ×
=
−×
2
OUTF INMIN SW OUTF IN
DC
2
OUTF INMIN OUTF
2 (V V ) f V L
G
(2V V ) I
The loop-compensation values for the error amplifier can
be calculated as:
( )
( )
−
−
××
=
π ××
OUTF INMIN OUTF
P
OUTF INMIN OUTF OUTF
2V V I
f
2V V V C
where V
INMIN
is the minimum operating input voltage
and I
OUTF
is the maximum load current.

××

= ×+ ×+




2
OUTF SW
Z
P
230 V 0.1 f
ms
R 11
GDC f mp
where ms = default slope compensation and mp =
V
INMIN
/L x 0.5.