Rainbow Electronics MAX15001 - Manual (Page 10)

MAX15000/MAX15001
threshold), then startup has been accomplished and
sustained operation will commence. If V
IN drops below
9.74V before startup is complete, the device goes back
to low-current UVLO. In this case, increase the value of
C1 to store enough energy to allow for the voltage at
the tertiary winding to build up.
UVLO Flag (UFLG)
The MAX15000/MAX15001 have an open-drain under-
voltage flag output (UFLG). When used with an opto-
coupler the UFLG output can serve to sequence a
secondary-side controller. An internal 210µs delay
occurs the instant the voltage on UVLO/EN drops
below 1.17V until NDRV stops switching. This allows for
the UFLG output to change state before the MAX15000/
MAX15001 shut down (Figure 3).
When the voltage at the UVLO/EN is above the thresh-
old, UFLG is high impedance. When UVLO/EN is below
the threshold, UFLG goes low. UFLG is not affected by
bootstrap UVLO (MAX15000).
Soft-Start
The MAX15000/MAX15001 soft-start feature allows the
output voltage to ramp up in a controlled manner, elimi-
nating voltage overshoot. The MAX15000/MAX15001
reference generator that is internally connected to the
error amplifier soft-starts to achieve superior control of
the output voltage under heavy and light load condi-
tions. Soft-start begins after UVLO is deasserted (V
IN is
above 21.6V for the MAX15000, V IN is above 9.5V for
the MAX15001, and the voltage on UVLO/EN is above
1.23V). The voltage applied to the noninverting node of
the amplifier ramps from 0 to 1.23V in 1984 NDRV
switching cycles. Use the following formula to calculate
the soft-start time (t
SS ):
where f NDRV is the switching frequency at the NDRV
output. Figure 4 shows the soft-start regulated output of
a power supply using the MAX15000 during startup.
t
f
SS
NDRV
=
1984
Current-Mode PWM Controllers with
Programmable Switching Frequency
10 ______________________________________________________________________________________
MAX15000 fig02
100ms/div
V
CC
2V/div
V IN
5V/div
0V
Figure 2. VIN and VCC During Startup When Using the
MAX15000 in Bootstrapped Mode (Figure 1)
V
UVLO/EN
LOW LOW
Hi-Z
V UFLG
V NDRV
SHUTDOWN SHUTDOWN
t EXTR
3ms
1.23V
( ±1%) 1.17V (typ)
t EXTF
210 µs
0.6 µs
3 µs
NDRV SWITCHING
Figure 3. UVLO/EN and UFLG Operation Timing