MAX5953A MAXIM [Maxim Integrated Products], MAX5953A Datasheet - Page 18

• Manufacturer Part Number: MAX5953A
• Description: IEEE 802.3af PD Interface and PWM Controllers with Integrated Power MOSFETs
• Manufacturer: MAXIM [Maxim Integrated Products]

Program the MAX5953A/MAX5953B/MAX5953C/
MAX5953D soft-start with an external capacitor (C
connected between CSS and GND. When the device
turns on, C
33µA, ramping up to 7.3V. During this time, the feed-
back input (OPTO) is clamped to V
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.
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
(14pF, typ), V
f
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
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:
IEEE 802.3af PD Interface and PWM Controllers
with Integrated Power MOSFETs
18
S
R
is the switching frequency.
RTCT
______________________________________________________________________________________
FLTINT
PCB
≅
2
f C
CSS
s
is the stray capacitance on the PC board
(
TH,RTCT
RTCT
reaches the shutdown threshold, the
charges with a constant current of
Integrating Fault Protection
+
is the RTCT peak trip level, and
C
PCB
)
In
⎛
⎜
⎝
1
V
REGOUT
CSS
V
REGOUT
+ 0.6V. This ini-
−
Oscillator
Soft-Start
V
TH RTCT
,
CSS
⎞
⎟
⎠
)
where I
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
Choose t
from a few hundred microseconds to a few milliseconds.
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.
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
where I
current.
To reduce switching noise, connect CS to an external
RC lowpass filter for additional filtering (Figure 3).
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.
SENSE
RT
FLTIN
, according to the following equation:
LimPrimary
RT
is the desired recovery time.
≥ 10 x t
R
R
is typically 80µA, and t
SENSE
FLTINT
Applications Information
C
FLTINT
is the maximum peak primary-side
SH
Current-Sense Comparator
= 0.156V / I
≅
. Typical values for t
C
≅
FLTINT
I
FLTINT
t
RT
1 4 .
× 0 3514
LimPrimary
×
Design Example
.
t
SH
SH
is the desired
SH
Shutdown
can range