LTC3707IGN datasheet(21/32 Pages) LINER

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LTC3707

3707fb

APPLICATIONS INFORMATION

but less than in normal operation. The short-circuit ripple

current is determined by the minimum on-time tON(MIN)

of the LTC3707 (less than 200ns), the input voltage and

inductor value:

ΔIL(SC) = tON(MIN) (VIN/L)

The resulting short-circuit current is:

SC =

25mV

SENSE

ΔI

L(SC)

Fault Conditions: Overvoltage Protection (Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the 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 control-

ler is operating.

A comparator monitors the output for overvoltage con-

ditions. The comparator (0V) 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 overvoltage

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 0V condition persists; if VOUT returns to a 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 regulate

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

controllers 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 (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 LTC3707 has an internal voltage 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 approximately 220kHz. The

FREQSET pin can be grounded to lower this frequency to

approximately 140kHz or tied to the INTVCC pin to yield

approximately 310kHz. The FREQSET pin may be driven

with a voltage from 0 to INTVCC to ﬁx or modulate the

oscillator frequency as shown in Figure 5.

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest time duration

that the LTC3707 is capable of turning on the top MOSFET.

It is determined by internal timing delays and the gate

charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that

ON(MIN) <

OUT

IN(f)

If the duty cycle falls below what can be accommodated

by the minimum on-time, the LTC3707 will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The typical tested minimum on-time of the LTC3707 is

180ns under an ideal condition without switching noise.

However, the minimum on-time can be affected by PCB

switching noise in the voltage and current loops. With

reasonably good PCB layout, minimum 30% inductor

current ripple and about 15mV sensing ripple voltage,

300ns minimum on-time is a conservative number to

start with.

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