Rainbow Electronics MAX5953D - Manual (Page 18)

MAX5953A/MAX5953B/MAX5953C/MAX5953D
Soft-Start
Program the MAX5953A/MAX5953B/MAX5953C/
MAX5953D soft-start with an external capacitor (C
CSS
)
connected between CSS and GND. When the device
turns on, C
CSS charges with a constant current of
33µA, ramping up to 7.3V. During this time, the feed-
back input (OPTO) is clamped to V
CSS + 0.6V. This ini-
tially holds the duty cycle lower than the value the
regulator imposes, thus preventing voltage overshoot at
the output. When the IC turns off, the soft-start capaci-
tor internally discharges to GND.
Oscillator
The oscillator is externally programmable through a
resistor connected from RTCT to REGOUT and a
capacitor connected from RTCT to GND. The PWM fre-
quency is one-half the frequency seen at RTCT with a
50% duty cycle. Use the following formula to calculate
the oscillator components:
where C PCB is the stray capacitance on the PC board
(14pF, typ), V TH,RTCT is the RTCT peak trip level, and
f S is the switching frequency.
Integrating Fault Protection
The integrating fault protection feature allows the IC to
ignore transient overcurrent conditions for a program-
mable amount of time, giving the power-supply time to
behave like a current source to the load. This can hap-
pen, for example, under load-current transients when
the control loop requests maximum current to keep the
output voltage from going out of regulation. The ignore
time is programmed externally by connecting a capaci-
tor from FLTINT to GND. Under sustained overcurrent
faults, the voltage across this capacitor ramps up
toward the FLTINT shutdown threshold (2.7V, typ).
When V
FLTINT reaches the shutdown threshold, the
power supply shuts down. A high-value bleed resistor
connected in parallel with the FLTINT capacitor allows
the capacitor to discharge toward the restart threshold
(1.9V, typ). FLTINT drops to the restart threshold allow-
ing for soft-starting the supply again.
The fault integration circuit works by forcing an 80µA
current into FLTINT for one clock cycle every time the
current-limit comparator ILIM (Figure 9) trips. Use the
following formula to calculate the approximate capaci-
tor needed for the desired shutdown time:
where I FLTIN is typically 80µA, and t SH is the desired
ignore time during which current-limit events from the
current-limit comparator are ignored.
This is an approximate formula; some testing may be
required to fine tune the actual value of the capacitor.
Calculate the approximate bleed resistor needed for
the desired recovery time using the following formula:
where t RT is the desired recovery time.
Choose t RT
≥ 10 x t
SH . Typical values for t SH can range
from a few hundred microseconds to a few milliseconds.
Shutdown
Shut down the controller section of the IC by driving
DCUVLO to GND using an open-collector or open-drain
transistor connected to GND. The DC-DC converter sec-
tion shuts down if REGOUT is below its DCUVLO level.
Current-Sense Comparator
The current-sense (CS) comparator and its associated
logic limit the peak current through the internal MOSFET.
Current is sensed at CS as a voltage across a sense
resistor between the source of the MOSFET and GND.
The power MOSFET switches off when the voltage at
CS reaches 156mV. Select the current-sense resistor,
R
SENSE , according to the following equation:
R SENSE = 0.156V / I LimPrimary
where I LimPrimary is the maximum peak primary-side
current.
To reduce switching noise, connect CS to an external
RC lowpass filter for additional filtering (Figure 3).
Applications Information
Design Example
Design Example 1: PD with three-output flyback DC-
DC converter
Figure 6 shows an isolated three-output flyback DC-DC
converter. It provides output voltages of 10V at 30mA,
5.1V at 1.8A, and 2.55V at 5.4A.
Design Example 2: PD with nonisolated step-down
(buck) converter
Figure 7 shows a buck converter with 12V, 0.75A out-
put. Caution: this converter does not have active cur-
rent limit.
R
t
C
FLTINT
RT
FLTINT
≅
× 0 3514
.
C
I t
FLTINT
FLTINT SH ≅
×
1 4 .
R
f C C In
V
V V
RTCT
s RTCT PCB
REGOUT
REGOUT TH RTCT
≅
+
( )
⎛
⎝
⎜
⎞
⎠
⎟
−
1
2
,
IEEE 802.3af PD Interface and PWM Controllers
with Integrated Power MOSFETs
18 ______________________________________________________________________________________