LT4276
14
4276fa
For more information www.linear.com/LT4276
applicaTions inForMaTion
In the absence of an output load current, the Iport_inrush,
is approximated by the following equation:
Iport_inrush ≈ (C
OUT
• V
OUT
2
)/(η • t
SFST
• V
IN
)
where η is the power supply efficiency,
V
IN
is the input voltage of the PD
Iport_inrush plus the port current due to the load current
must be below the current the PSE can provide. Note that
the PSE current capability depends on the PSE operating
standard.
The LT4276 contains a soft-start function that controls
t
SFST
by connecting an external capacitor, C
SFST
, between
the SFST pin and GND. The SFST pin is pulled up with I
SFST
when the LT4276 begins switching. The voltage ramp on
the SFST pin is proportional to the duty cycle ramp for PG.
For flyback mode, the soft-start time is:
t
SFST
=
600µA
nF
C
SFST
I
SFST
⎛
⎝
⎜
⎞
⎠
⎟
t
PGon
+ t
PGDELAY
– t
MIN
( )
where t
PGon
is the time when PG is high as shown in
Figure 8 once the power supply is in steady-state.
In for
ward mode, each of the back page applications sche-
matics provides a chart with t
SFST
vs. C
SFST
. Select the
application and choose a value of C
SFST
that corresponds
to the desired soft-start time.
CURRENT SENSE COMPARATOR
The LT4276 uses a differential current sense comparator
to reduce the effects of stray resistance and inductance
on the measurement of the primary current. ISEN
+
and
ISEN
–
must be Kelvin connected to the sense resistor pads.
Like most switching regulator controllers, the current
sense comparator begins sensing the current t
MIN
after
PG turns on. Then, the comparator turns PG off after the
voltage across ISEN
+
and ISEN– exceeds the current
sense comparator threshold, V
SENSE
. Note that the voltage
across ISEN
+
and ISEN– is modified by LT4276’s internal
slope compensation.
SLOPE COMPENSATION
The LT4276 incorporates current slope compensation.
Slope compensation is required to ensure current loop
stability when the duty cycle is greater than or near 50%.
The slope compensation of the LT4276 does not reduce
the maximum peak current at higher duty cycles.
CONTROL LOOP COMPENSATION
In flyback mode, loop frequency compensation is per
-
formed by connecting a resistor/capacitor network from
the output
of the feedback amplifier (ITHB pin) to GND as
shown in Figure 12. In forward mode, loop compensation
is performed by varying R
X
and C
X
in Figure 14.
ADJUSTABLE SWITCHING FREQUENCY
The LT4276 has a default switching frequency, f
OSC
, of 214
kHz when the ROSC pin is left open. If a higher switching
frequency, f
SW
, is desired (up to 300 kHz), a resistor no
smaller than 45.3kΩ may be added between the ROSC pin
to GND. The resistor can be calculated below:
R
OSC
=
f
SW
– f
OSC
kΩ
( )
SHORT CIRCUIT RESPONSE
If the power supply output voltage is shorted, overloaded,
or if the soft-start capacitor is too small, an overcurrent
fault event occurs when the voltage across the sense pins
exceeds V
FAULT
(after the blanking period of t
MIN
). This
begins the internal fault timer t
FAULT
. For the duration
of t
FAULT
, the LT4276 turns off PG and SG and pulls the
SFST pin to GND. After t
FAULT
expires, the LT4276 initi-
ates soft-start.
The fault and soft-start sequence repeats as long as the
short circuit or overload conditions persist. This condition
is recognized by the PG waveform shown in Figure 15
re
peating at an interval of t
FAULT
.
Figure 15: PG Waveform with Output Shorted
t
FAULT
4276 F14
