MAX17498A(2012) Ver la hoja de datos (PDF) - Maxim Integrated
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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
IN
CIN
IN
VCC
LDO
MAX17498B
MAX17498C LX
VDC
VOUT
D1
CVCC
Np
Ns
COUT
Figure 5. MAX17498B/MAX17498C Typical Startup Circuit with
IN Connected Directly to DC Input
VDC
RZ
Q1
ZD1
6.3V
IN
CIN
CVCC
D2
MAX17498B
MAX17498C
IN
LDO
NB
LX
Np
VOUT
D1
COUT
Ns
VCC
Figure 6. MAX17498B/MAX17498C Typical Startup Circuit with
Bias Winding to Turn Off Q1 and Reduce Power Dissipation
The startup capacitor (CSTART) can be calculated as:
C START
= IIN + QGA1T0E6×
fSW
×
t SS
10
µF
where IIN is the supply current drawn at the IN pin in mA,
QGATE is the gate charge of the external MOSFET used
in nC, fSW is the switching frequency of the converter in
kHz, and tSS is the soft-start time programmed for the
flyback converter in ms.
Resistors RSUM and RISRC can be calculated as:
= R SUM
VSTART MΩ
10
= RISRC
VBEQ1 MΩ
70
The IN UVLO wakeup threshold of the MAX17498B/
MAX17498C is set to 3.9V (typ) with a 200mV hyster-
esis, optimized for low-voltage DC-DC applications
down to 4.5V. For applications where the input DC
voltage is low enough (e.g., 4.5V to 5.5V DC) that the
power loss incurred to supply the operating current of
the MAX17498B/MAX17498C can be tolerated, the IN
pin is directly connected to the DC input, as shown in
Figure 5. In the case of higher DC input voltages (e.g., 16V
to 32V DC), a startup circuit, such as that shown in Figure 6,
can be used to minimize power dissipation in the startup
circuit. In this startup scheme, the transistor (Q1)
supplies the switching current until a bias winding NB
comes up. The resistor (RZ) can be calculated as:
RZ = 9 × (VINMIN − 6.3) kΩ
where VINMIN is the minimum input DC voltage.
Programming Soft-Start of the
Flyback/Boost Converter (SS)
The soft-start period in the voltage soft-start scheme of
the devices can be programmed by selecting the value
of the capacitor connected from the SS pin to GND.
The capacitor CSS can be calculated as:
C= SS 8.13 × t SS nF
where tSS is expressed in ms.
The soft-start period in the current soft-start scheme
depends on the load at the output and the soft-start
capacitor.
Programming Output Voltage
The devices incorporate a flexible error amplifier that
allows regulating to both the positive and negative
outputs. The positive output voltage of the converter
can be programmed by selecting the correct values
for the resistor-divider connected from VOUT, the fly-
back /boost output to ground, with the midpoint of the
divider connected to the EA- pin (Figure 7). With RB
selected in the range of 20kI to 50kI, RU can be
calculated as:
RU
=
RB
×
VOUT
1.22
− 1 kΩ
where RB is in kI.
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