MAX17498A_13 Datasheet, PDF (13/20 Pages) Maxim Integrated Products – AC-DC and DC-DC Peak Current-Mode Converters for Flyback/Boost Applications

MAX17498A/MAX17498B/MAX17498C
AC-DC and DC-DC Peak Current-Mode Converters
for Flyback/Boost Applications
IN
CIN
IN
V_CC
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
Resistors R_SUM and R_ISRC can be calculated as:
= R _SUM
V_{START MΩ}
10
= R_ISRC
V_{BEQ1 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
Maxim Integrated
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 dis-
sipation. In this startup scheme, the transistor (Q1)
supplies the switching current until a bias winding NB
comes up and turns off Q1. The resistor (R_Z) can be
calculated as:
R_Z = 2 × (V_INMIN − 6.3) kΩ
where V_INMIN 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 C_SS can be calculated as:
C= _SS 8.13 × t _SS nF
where t_SS 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 V_OUT, the fly-
back /boost output to ground, with the midpoint of the
divider connected to the EA- pin (Figure 7). With R_B
selected in the range of 20kI to 50kI, R_U can be
calculated as:
R_U
=
R_B
×
V_OUT
 _1.22
− 1^{} kΩ
where R_B is in kI.
The negative output voltage of the converter can be
programmed by selecting the correct values for the
resistor-divider connected from VOUT, the flyback /boost
output to REF with the midpoint of the divider connected
to the EA+ pin (Figure 8). With R1 selected in the range
of 20kI to 50kI, R2 can be calculated as:
R2
= R1× ^{V1O}_.2^U₂^T


kΩ
where R1 is in kI.
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