MAX17498A(2012) Ver la hoja de datos (PDF) - Maxim Integrated
Número de pieza
componentes Descripción
Fabricante
MAX17498A
(Rev.:2012)
(Rev.:2012)
Maxim Integrated
MAX17498A Datasheet PDF : 30 Pages
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MAX17498A / MAX17498B / MAX17498C
AC-DC and DC-DC Peak Current-Mode Converters
for Flyback/Boost Applications
Input-Capacitor Selection
The value of the required input ceramic capacitor 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 calculations are:
CIN
=
VINMIN
3.75 × IOUT
× fSWMIN × (1−
D MAX )
The capacitor RMS can be calculated as:
ICIN_RMS
=
IPK
2×
3
Error-Amplifier Compensation Design
The loop-compensation values for the error amplifier can
now be calculated as:
= C Z
GDC × GM= × 10
2 × π × fSW
(GDC ×10) nF
where GDC, the DC gain of the power stage, is given as:
GDC =
8 × (VOUT − VINMIN) × fSW × VOUT 2 × LIN
(2VOUT − VINMIN)2 × IOUT
RZ
=
VOUT × COUT × (VOUT − VINMIN)
IOUT × CZ × (2VOUT − VINMIN)
where VINMIN is the minimum operating input voltage
and IOUT is the maximum load current:
CP
=
COUT × ESR
RZ
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 SLOPE pin to the
VCC 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
capacitances in the circuit interact to produce volt-
age overshoot 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 rating of 1.3 x VOUT provides the
necessary design margin in most cases.
The current rating of the output diode should be selected
so that the power loss in the diode (given as the product
of forward-voltage drop and the average diode current)
is low enough to ensure that the junction temperature
is within limits. This necessitates that the diode current
rating be in the order of 2 x IOUT to 3 x IOUT. 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 LX, ideally equals the sum of the output
voltage and the forward drop of the output diode. In
practice, voltage overshoot and ringing occur due to the
action of circuit parasitic elements during the turn-off
transition. The maximum rating of the devices’ internal
n-channel MOSFET is 65V, making it possible to design
boost converters with output voltages up to 48V and suffi-
cient margin for voltage overshoot and ringing. The RMS
current into LX is useful in estimating the conduction loss
in the internal nMOSFET, and is given as:
ILX_RMS =
IPK 3 × LINS × fSW
3 × VINMIN
where IPK is the peak current calculated at the lowest
operating input voltage (VINMIN).
CCM Boost
In a CCM boost converter, the inductor current does
not return to zero during a switching cycle. Since the
MAX17498B 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:
LIN
=
VIN × D × (1− D)
0.3 × IOUT × fSW
where D is the duty cycle calculated as:
D = VOUT + VD − VIN
VOUT + VD
VD is the voltage drop across the output diode of the
boost converter at maximum output current.
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