Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
24Maxim Integrated
=
π× ×
=
π× ×
Z
PZ
P
SW Z
1
C
2fR
1
C
fR
Slope Compensation
In theory, the DCM boost converter does not require
slope compensation for stable operation. In practice, the
converter needs a minimum amount of slope for good
noise immunity at very light loads. The minimum slope
is set for the devices by connecting the SCOMPF pin to
the V
CC
pin.
Output Diode Selection
The voltage rating of the output diode for the boost
converter ideally equals the output voltage of the boost
converter. In practice, parasitic inductances and capaci-
tances in the circuit interact to produce voltage over-
shoot during the turn-off transition of the diode that
occurs when the main switch turns on. The diode rating
should therefore be selected with the necessary margin
to accommodate this extra voltage stress. A voltage rat-
ing of 1.3 x V
OUTF
provides the necessary design margin
in most cases.
The current rating of the output diode should be selected
such that the power loss in the diode (given as the
product of forward-voltage drop and the average diode
current) should be low enough to ensure that the junction
temperature is within limits. This necessitates the diode
current rating to be in the order of 2 x I
OUTF
to 3 x I
OUTF
.
Select fast-recovery diodes with a recovery time less than
50ns, or Schottky diodes with low junction capacitance.
Internal MOSFET RMS Current Calculation
The voltage stress on the internal MOSFET, whose drain
is connected to LXF, ideally equals the sum of the out-
put voltage and the forward drop of the output diode.
In practice, voltage overshoot and ringing occur due
to action of circuit parasitic elements during the turn-off
transition. The maximum rating of the internal nMOSFET
of the devices is 65V, making it possible to design boost
converters with output voltages up to 48V, with sufficient
margin for voltage overshoot and ringing. The RMS
current into LXF is useful in estimating the conduction
loss in the internal nMOSFET and is given as:
3
PK INS SW
LXF_RMS
INMIN
I Lf
I
3V
××
=
×
where I
PK
is the peak current calculated at the lowest
operating input voltage (V
INMIN
).
CCM Boost
In a CCM boost converter, the inductor current does
not return to zero during a switching cycle. Since the
MAX17497B implements a nonsynchronous boost con-
verter, the inductor current enters DCM operation at load
currents below a critical value, equal to 1/2 the peak-to-
peak ripple in the inductor current.
Inductor Selection
The design procedure starts with calculating the boost
converter’s input inductor at nominal input voltage for a
ripple in the inductor current, equal to 30% of the maxi-
mum input current:
IN
IN
OUTF SW
V D (1 D)
L
0.3 I f
××−
=
××
where D is the duty cycle calculated as:
OUTF D IN
OUTF D
V VV
D
VV
+−
=
+
V
D
is the voltage drop across the output diode of the
boost converter at maximum output current.
Peak/RMS Current Calculation
To set the current limit, the peak current in the inductor
and internal nMOSFET can be calculated as:

× ×−
= +×

×−

<
OUTF MAX MAX OUTF
PK
IN SW MAX
MAX
V D (1 D ) I
I 1.2
L f (1 D )
for D 0.5

×
= +× ≥

×−

OUTF OUTF
PK MAX
IN SW MAX
0.25 V I
I 1.2 for D 0.5
L f (1 D )
D
MAX
, the maximum duty cycle, is obtained by substitut-
ing the minimum input operating voltage (V
INMIN
) in the
equation above for duty cycle. L
INMIN
is the minimum