Rainbow Electronics MAX5051 - Manual (Page 17)

Integrating Fault Protection
The integrating fault protection feature allows transient
overcurrent conditions to be ignored for a programmable
amount of time, giving the power supply time to behave
like a current source to the load. This can
happen, for example, under load-current transients when
the control loop requests maximum current to keep the
output voltage from going out of regulation. The fault
integration time can be programmed externally by con-
necting a suitably sized capacitor to the FLTINT pin.
Under sustained overcurrent faults, the voltage across
this capacitor is allowed to ramp up towards the FLTINT
shutdown threshold (2.9V, typ). Once the threshold is
reached, the power supply shuts down. A high-value
bleed resistor connected in parallel with the FLTINT
capacitor allows it to discharge towards the restart
threshold (1.8V, typ). Once this threshold is reached, the
supply restarts with a new soft-started cycle.
Note that cycle-by-cycle current limiting is provided at
all times by CS with a threshold of 154mV (typ). The
fault integration circuit works by forcing a 90µA current
out of FLTINT every time that the current-limit compara-
tor (Figure 1, CILIM) is tripped. Use the following formu-
la to calculate the value of the capacitor necessary for
the desired shutdown time of this circuit.
where I FLTINT = 90µA, t SH is the desired fault integra-
tion time after the first shutdown cycle during which
current-limit events from the current-limit comparator
are ignored. For example, a 0.1µF capacitor gives a
fault integration time of 2.25ms.
Some testing may be required to fine-tune the actual
value of the capacitor. To calculate the required bleed
resistance R
FLTINT , use the following formula:
where t RT is the desired recovery time.
Typically choose t RT = 10 x t SH . Typical values for t SH
range from a few hundred microseconds to a few mil-
liseconds.
Synchronizing Primary-Side STARTUP For
Parallel Operation
Figure 7 shows the connection diagram of two or more
MAX5051s for synchronized primary-side operation.
The common connection of STARTUP ensures all paral-
leled modules wakeup and shutdown in tandem. This
helps prevent startup conflicts when the secondaries of
the power supplies are paralleled. Connecting
SYNCOUT to SYNCIN is not necessary; however, when
used, this minimizes the ripple current though the input
bypass capacitors.
Applications Information
Isolated Telecom Power Supply
Figure 8 shows a complete design of an isolated syn-
chronously rectified power supply with a 36V to 72V
telecom voltage range. This power supply is fully pro-
tected and can sustain a continuous short circuit at its
output terminals. Figures 9 though 14 show some of the
performance aspects of this power-supply design. This
circuit is available as a completely built and tested
evaluation kit.
R
t
C
FLTINT
RT
FLTINT
=
×
0 372 .
C
I t
V
FLTINT
FLTINT SH =
. 0 9
MAX5051
Parallelable, Clamped Two-Switch
Power-Supply Controller IC
______________________________________________________________________________________ 17
RCOSC
CON
RCFF
SYNCIN
UVLO
STARTUP
SYNCOUT FLTINT
RCOSC
CON
RCFF
SYNCIN
UVLO
STARTUP
SYNCOUT FLTINT
#1
#2
MAX5051
MAX5051
Figure 7. Connection for Synchronized STARTUP of Two or
More MAX5051s