LTC1876
22
1876fa
The value of the soft-start capacitor C
SS
may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C
SS
> (C
OUT
)(V
OUT
) (10
–4
) (R
SENSE
)
The minimum recommended soft-start capacitor of
C
SS
= 0.1µF will be sufficient for most applications.
Fault Conditions: Current Limit and Current Foldback
The LTC1876 step-down controllers current comparator
has a maximum sense voltage of 75mV resulting in a
maximum MOSFET current of 75mV/R
SENSE
. The maxi-
mum value of current limit generally occurs with the
largest V
IN
at the highest ambient temperature, conditions
that cause the highest power dissipation in the top MOSFET.
The controllers include 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 step-down regulators 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
ON(MIN)
(less than 200ns), the input voltage and
inductor value:
∆I
L(SC)
= t
ON(MIN)
(V
IN
/L)
The resulting short-circuit current is:
I
mV
R
I
SC
SENSE
LSC
=+
25 1
2
∆
()
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the step-down
regulator rises much higher than nominal levels. The
crowbar causes huge currents to flow, that blow the fuse
to protect against a shorted top MOSFET if the short
occurs while the controller is operating.
A comparator monitors the output for overvoltage condi-
tions. The comparator (OV) detects overvoltage faults
greater than 7.5% above the nominal output voltage.
When this condition is sensed, the top MOSFET is turned
off and the bottom MOSFET is turned on until the overvolt-
age condition is cleared. The output of this comparator is
only latched by the overvoltage condition itself and will
therefore allow a switching regulator system having a poor
PC layout to function while the design is being debugged.
The bottom MOSFET remains on continuously for as long
as the OV condition persists; if V
OUT
returns to safe level,
normal operation automatically resumes. A shorted top
MOSFET will result in a high current condition which will
open the system fuse. The switching regulator will regu-
late properly with a leaky top MOSFET by altering the duty
cycle to accommodate the leakage.
The Standby Mode (STBYMD) Pin Function
The Standby Mode (STBYMD) pin provides several choices
for start-up and standby operational modes. If the pin is
pulled to ground, the RUN/SS pins for both controllers are
internally pulled to ground, preventing start-up and thereby
providing a single control pin for turning off both control-
lers at once. If the pin is left open or decoupled with a
capacitor to ground, the RUN/SS pins are each internally
provided with a starting current enabling external control
for turning on each controller independently. If the pin is
provided with a current of >3µA at a voltage greater than
2V, both internal linear regulators (INTV
CC
and 3.3V) will
be on even when both controllers are shut down. In this
mode, the onboard 3.3V and 5V linear regulators can
provide power to keep-alive functions such as a keyboard
controller. This pin can also be used as a latching “on” and/
or latching “off” power switch if so designed.
Frequency of Operation
The LTC1876 stepdown controllers have an internal volt-
age controlled oscillator. The frequency of this oscillator
can be varied over a 2 to 1 range. The pin is internally self-
biased at 1.19V, resulting in a free-running frequency of
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