LT8310
20
8310f
For more information www.linear.com/LT8310
applicaTions inForMaTion
applications operating without output voltage feedback,
current sense information is fed back to the duty cycle loop
to reduce output voltage ringing due to load current steps
that excite the output LC tank. In supply applications, each
cycle ends when the amplified SENSE voltage exceeds the
V
C
pin control level. In all cases, during the cycle on-time,
the switch sees the rippling inductor current (I
L1
), scaled
by the transformer turns ratio (Equation 18) plus the
transformer’s primary magnetizing current, I
µ,p
. Applying
V
IN
across the magnetizing inductance generates a peak
magnetizing current of approximately 12 • V
SET
• t
SW
/L
µ,p
.
I
SWITCH
=
L1
N
P
/N
S
+I
µ,p
[18]
Resistor R
SENSE
connected between the SENSE and GND
pins converts the switch current to a voltage. It should be
selected to provide the maximum switch current required
by the application, including inductor ripple current, without
exceeding the SENSE pin’s overcurrent threshold. A good
rule of thumb is to allow 10% margin on the minimum
overcurrent threshold of 115mV.
During steady-state operation, the average inductor current
equals the load current. In applications under duty mode
control, which require a minimum load, less inductor ripple
means a lower minimum load current, so peak inductor
current might be 10% or less above the maximum load
current. Output voltage ring damping operates best with a
strong average current signal, so R
SENSE
should be chosen
as large as allowed by the SENSE pin threshold. Equation
19 provides a good value for R
SENSE
that accounts for the
minimum SENSE threshold:
R
SENSE
≤
1.1• I
SWITCH(MAX)
[19]
In applications with output voltage feedback, current mode
control is most agile with a steep slope to the ripple, so
peak inductor current might be 20% or more above the
average load current. Equation 20 provides a good value for
R
SENSE
that accounts for the minimum SENSE threshold:
R
SENSE
≤
1.4 • I
SWITCH(MAX)
[20]
It is always prudent to verify the peak inductor current
in the application to ensure the sense resistor selection
provides margin to the SENSE overcurrent limit threshold.
The placement of R
SENSE
should be close to the source
of the N-channel MOSFET and GND of the LT8310. The
SENSE input to LT8310 should be a Kelvin connection to
the positive terminal of R
SENSE
. Verify the power in the
resistor to ensure that it does not exceed its rated maximum.
Programming the Soft-Start Interval and Hiccup
Period
The built-in soft-start circuit significantly reduces the inrush
current spike and output voltage overshoot at start-up.
Please refer to Figure 6 and the Timing Diagrams section
for the following discussion of soft-start behavior. The
soft-start interval is programmed by a capacitor connected
from the SS pin to GND. In a normal start-up, after the
INTV
CC
voltage exceeds its rising threshold of about
5.2V, the SS pin sources 50µA (typical), which ramps the
capacitor voltage. Switching commences when the 1.00V
switching threshold is exceeded (EN_GATE high).
Assuming the SS pin starts fully discharged, the soft-start
time, t
SS
, may be programmed by choosing C
SS
using
Equation 21. A 100nF soft-start capacitor produces about
2ms of delay, which suits many applications.
C
SS
= 50nF •
SS
[ ]
1ms
[21]
The SS pin voltage is discharged when the fault latch is
set under any of the following conditions: the UVLO pin
voltage falls below its threshold (SYS_UV high), the OVLO
pin voltage exceeds its threshold (SYS_OV high), the die
temperature exceeds 165°C (SYS_OT high), the INTV
CC
voltage falls below or rises above its operating range
(REG_UV or REG_OV high), or the SENSE pin voltage
exceeds its maximum threshold because the switch current
is too large (ISW_MAX high). When the fault condition
ceases and V
SS
< 0.27V, the fault latch clears, which brings
about restart as SS rises through the 1V threshold.
Exceeding maximum switch current sets the hiccup latch,
which extends the soft-start time by reducing the pull-up
current to 5µA (typical). After the fault latch is reset, the
