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LTC3780

3780fe

APPLICATIONS INFORMATION

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 SS pin is internally pulled to ground,

preventing start-up and thereby providing a single control

pin for turning off the controller. If the pin is left open or

bypassed to ground with a capacitor, the SS pin is internally

provided with a starting current, permitting external control

for turning on the controller. If the pin is connected to a

voltage greater than 1.25V, the internal regulator (INTVCC)

will be on even when the controller is shut down (RUN

pin voltage < 1.5V). In this mode, the onboard 6V linear

regulator can provide power to keep-alive functions such

as a keyboard controller.

Fault Conditions: Current Limit and Current Foldback

The maximum inductor current is inherently limited in a

current mode controller by the maximum sense voltage.

In boost mode, maximum sense voltage and the sense

resistance determines the maximum allowed inductor

peak current, which is:

IL(MAX,BOOST) =

160mV

RSENSE

In buck mode, maximum sense voltage and the sense

resistance determines the maximum allowed inductor

valley current, which is:

IL(MAX,BUCK) =

130mV

RSENSE

To further limit current in the event of a short circuit to

ground, the LTC3780 includes foldback current limiting.

If the output falls by more than 30%, then the maximum

sense voltage is progressively lowered to about one third

of its full value.

Fault Conditions: Overvoltage Protection

A comparator monitors the output for overvoltage con-

ditions. The comparator (OV) detects overvoltage faults

greater than 7.5% above the nominal output voltage.

When the condition is sensed, switches A and C are

turned off, and switches B and D are turned on until the

overvoltage condition is cleared. During an overvoltage

condition, a negative current limit (VSENSE = –60mV) is

set to limit negative inductor current. When the sensed

current inductor current is lower than –60mV, switch A and

C are turned on, and switch B and D are turned off until

the sensed current is higher than –20mV. If the output is

still in overvoltage condition, switch A and C are turned

off, and switch B and D are turned on again.

Efﬁciency Considerations

The percent efﬁciency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efﬁciency and which change would

produce the most improvement. Although all dissipative

elements in circuit produce losses, four main sources

account for most of the losses in LTC3780 circuits:

1. DC I2R losses. These arise from the resistances of the

MOSFETs, sensing resistor, inductor and PC board

traces and cause the efﬁciency to drop at high output

currents.

2. Transition loss. This loss arises from the brief amount

of time switch A or switch C spends in the saturated

region during switch node transitions. It depends upon

the input voltage, load current, driver strength and

MOSFET capacitance, among other factors. The loss

is signiﬁcant at input voltages above 20V and can be

estimated from:

Transition Loss ≈ 1.7A–1 • VIN2 • IOUT • CRSS • f

where CRSS is the reverse transfer capacitance.

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