Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
26Maxim Integrated
Internal MOSFET RMS Current Calculation
The voltage stress on the internal MOSFET, whose drain
is connected to LXF, ideally equals the sum of the output
voltage and the forward drop of the output diode. In prac-
tice, voltage overshoot and ringing occur due to action
of circuit parasitic elements during the turn-off transition.
The devices’ maximum rating of the internal nMOSFET
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:
OUTF MAX
LXF_RMS
MAX
ID
I
(1 D )
×
=
−
where D
MAX
is the duty cycle at the lowest operating
input voltage and I
OUTF
is the maximum load current.
Thermal Considerations
It should be ensured that the junction temperature of the
devices does not exceed +125NC under the operating
conditions specified for the power supply. The power dis-
sipated in the devices to operate can be calculated using
the following equation:
IN IN IN
P VI= ×
where V
IN
is the voltage applied at the IN pin and I
IN
is
operating supply current.
The internal nMOSFET experiences conduction loss and
transition loss when switching between on and off states.
These losses are calculated as:
( )
2
CONDUCTION LXF_RMS DSON_LXF
TRANSITION INMAX PK R F SW
P IR
P 0.5 V I t t f
= ×
=× × ×+×
where t
R
and t
F
are the rise and fall times of the internal
nMOSFET in CCM operation. In DCM operation, because
the switch current starts from zero only, t
F
exists and the
transition loss equation changes to:
TRANSITION INMAX PK F SW
P 0.5 V I t f= × × ××
Additional loss occurs in the system in every switch-
ing cycle due to energy stored in the drain-source
capacitance of the internal MOSFET being lost when
the MOSFET turns on and discharges the drain-source
capacitance voltage to zero. This loss is estimated as:
2
CAP DS DSMAX SW
P 0.5 C V f=×× ×
The internal step-down regulator also has similar losses
that affect the temperature rise of the part. These losses
are estimated as:
(
)
2
LOSSBUCK OUTB DC
OUT
1
P P ( 1) I R= × −− ×
η
where E is the efficiency of the internal step-down
regulator at the output current (I
OUTB
), and R
DC
is the
DC resistance of the output inductor.
The total power loss in the devices can be calculated
from the following equation:
LOSS IN CONDUCTION TRANSITION CAP
LOSSBUCK
P PP P P
P
=+ ++
+
The maximum power that can be dissipated in the
devices is 1666mW at +70NC temperature. The power-
dissipation capability should be derated as the tem-
perature rises above +70NC at 21mW/NC. For a multilayer
board, the thermal-performance metrics for the package
are given below:
JA
48 C/Wθ=°
JC
10 C/Wθ=°
The junction temperature rise of the devices can be
estimated at any given maximum ambient temperature
(T
A_MAX
) from the following equation:
( )
J_MAX A_MAX JA LOSS
TT P= +θ ×
If the application has a thermal-management system that
ensures that the devices’ exposed pad is maintained at a
given temperature (T
EP_MAX
) by using proper heatsinks,
then the junction temperature rise can be estimated at
any given maximum ambient temperature from the fol-
lowing equation:
( )
J_MAX EP_MAX JC LOSS
TT P= +θ ×