ltc3728legn-trpbf Linear Technology Corporation, ltc3728legn-trpbf Datasheet - Page 22

• Manufacturer Part Number: ltc3728legn-trpbf
• Description: Dual, 550khz, 2-phase Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3728
started and been given adequate time to charge up the
output capacitor and provide full load current, the RUN/SS
capacitor is used for a short-circuit timer. If the regulator’s
output voltage falls to less than 70% of its nominal value
after C
sumption that the output is in an overcurrent condition. If
the condition lasts for a long enough period as determined
by the size of the C
the controller will be shut down until the RUN/SS pin volt-
age is recycled. If the overload occurs during start-up, the
time can be approximated by:
If the overload occurs after start-up the voltage on C
begin discharging from the zener clamp voltage:
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the RUN/SS pin as shown
in Figure 7. This resistance shortens the soft-start period
and prevents the discharge of the RUN/SS capacitor during
an over current condition. Tying this pull-up resistor to
V
connecting this pull-up resistor to INTV
7b, eliminates any extra supply current during controller
shutdown while eliminating the INTV
preventing controller start-up.
Why should you defeat overcurrent latchoff? During the
prototyping stage of a design, there may be a problem
with noise pickup or poor layout causing the protection
circuit to latch off. Defeating this feature will easily allow
troubleshooting of the circuit and PC layout. The internal
short-circuit and foldback current limiting still remains
active, thereby protecting the power supply system from
failure. After the design is complete, a decision can be
made whether to enable the latchoff feature.
22
IN
t
t
LO2
LO1
as in Figure 7a, defeats overcurrent latchoff. Diode-
SS
≈ [C
≈ [C
= 2.7 • 10
reaches 4.1V, C
SS
SS
(6 – 3.5)]/(1.2μA) = 2.1 • 10
(4.1 – 1.5 + 4.1 – 3.5)]/(1.2μA)
6
SS
(C
and the specifi ed discharge current,
SS
SS
)
begins discharging on the as-
CC
CC
6
loading from
, as in Figure
(C
SS
)
SS
will
The value of the soft-start capacitor C
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
The minimum recommended soft-start capacitor of
C
Fault Conditions: Current Limit and Current Foldback
The LTC3728 current comparator has a maximum sense
voltage of 75mV resulting in a maximum MOSFET cur-
rent of 75mV/R
limit generally occurs with the largest V
ambient temperature, conditions that cause the highest
power dissipation in the top MOSFET.
The LTC3728 includes current foldback to help further
limit load current when the output is shorted to ground.
The foldback circuit is active even when the overload
shutdown latch described above is overridden. If the
output falls below 70% of its nominal output level, then
the maximum sense voltage is progressively lowered from
75mV to 25mV. Under short-circuit conditions with very
low duty cycles, the LTC3728 will begin cycle skipping in
order to limit the short-circuit current. In this situation
the bottom MOSFET will be dissipating most of the power
but less than in normal operation. The short-circuit ripple
current is determined by the minimum on-time t
of the LTC3728 (less than 200ns), the input voltage and
inductor value:
The resulting short-circuit current is:
SS
C
ΔI
I
SC
SS
= 0.1μF will be suffi cient for most applications.
L(SC)
=
> (C
R
25mV
= t
SENSE
OUT
ON(MIN)
)(V
SENSE
+
OUT
2
1
(V
. The maximum value of current
) (10
I
IN
L(SC)
/L)
–4
) (R
SENSE
SS
)
IN
may need to be
at the highest
ON(MIN)
3728fd