MAX17499 Maxim Integrated Products, MAX17499 Datasheet - Page 13

• Manufacturer Part Number: MAX17499
• Description: (MAX17499 / MAX17500) Current-Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

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Choose a higher value for R1 than the one calculated in
the previous equation if a longer startup time can be
tolerated to minimize power loss on this resistor.
The above startup method is applicable to a circuit simi-
lar to the one shown in Figure 1. In this circuit, the ter-
tiary winding has the same phase as the output
windings. Thus, the voltage on the tertiary winding at any
given time is proportional to the output voltage and goes
through the same soft-start period as the output voltage.
The minimum discharge time of C1 from 21.6V to 9.74V
must be greater than the soft-start time of 5.6ms.
Figure 5. Secondary-Side Regulated, Isolated Power Supply
TRANS
OPTO
U2
V
IN
R6
______________________________________________________________________________________
R5
C1
R1
Programmable Switching Frequency
Current-Mode PWM Controllers with
R7
C2
IN
V
COMP
UFLG
FB
CC
R12
MAX17500A
RT
U1
C6
UVLO/EN
R15
NDRV
GND
D1
CS
Another method for bootstrapping the power supply is
to use a bias winding that is in-phase with the MOSFET
on-time (see Figure 5). In this case, the amount of
capacitance required at IN (C1) is much smaller.
However, the input voltage cannot have a range
greater than approximately 2:1 (primary-winding volt-
age to bias-winding voltage ratio).
For hiccup-mode fault protection, make the bias wind-
ing in-phase with the output, then the power-supply hic-
cups and soft-starts under output short-circuit
conditions. The power supply does not hiccup if the
bias winding is in-phase with the MOSFET on-time.
R3
R2
Q1
R4
T1
D2
OPTO LED
C4
TL431
U3
U2
R8
C3
R9
R10
V
OUT
13
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