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LTC1876

1876fa

The value of the soft-start capacitor CSS may need to be

scaled with output voltage, output capacitance and load

current characteristics. The minimum soft-start capaci-

tance is given by:

CSS > (COUT )(VOUT) (10–4) (RSENSE)

The minimum recommended soft-start capacitor of

CSS = 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/RSENSE. The maxi-

mum value of current limit generally occurs with the

largest VIN 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 tON(MIN) (less than 200ns), the input voltage and

inductor value:

∆IL(SC) = tON(MIN) (VIN/L)

The resulting short-circuit current is:

SENSE

LSC

∆ ()

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 VOUT 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 >3

µA at a voltage greater than

2V, both internal linear regulators (INTVCC 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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