MAX15001 Maxim Integrated Products, MAX15001 Datasheet - Page 10

• Manufacturer Part Number: MAX15001
• Description: (MAX15000 / MAX15001) Current-Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

threshold), then startup has been accomplished and
sustained operation will commence. If V
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.
Current-Mode PWM Controllers with
Programmable Switching Frequency
Figure 2. V IN and V CC During Startup When Using the
MAX15000 in Bootstrapped Mode (Figure 1)
Figure 3. UVLO/EN and UFLG Operation Timing
10
______________________________________________________________________________________
V
V
V
UVLO/EN
NDRV
UFLG
100ms/div
SHUTDOWN
LOW
( 1%)
1.23V
MAX15000 fig02
3 s
IN
t
3ms
EXTR
V
2V/div
V
5V/div
0V
drops below
CC
IN
Hi-Z
NDRV SWITCHING
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).
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
above 21.6V for the MAX15000, V
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
where f
output. Figure 4 shows the soft-start regulated output of
a power supply using the MAX15000 during startup.
0.6 s
NDRV
210 s
1.17V (typ)
t
EXTF
LOW
is the switching frequency at the NDRV
SS
t
SS
):
f
1984
NDRV
SHUTDOWN
UVLO Flag (UFLG)
IN
is above 9.5V for
Soft-Start
IN
is