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LTC1775

A reasonable starting point is to choose a ripple current

that is about 40% of IO(MAX). Note that the largest ripple

current occurs at the highest VIN. To guarantee that ripple

current does not exceed a specified maximum, the induc-

tor should be chosen according to:

OUT

L MAX

OUT

IN MAX

≥

















()(

)

–

()

∆

Burst Mode Operation Considerations

The choice of RDS(ON) and inductor value also determines

the load current at which the LTC1775 enters Burst Mode

operation. When bursting, the controller clamps the peak

inductor current to approximately:

BURST PEAK

DS ON

()

The corresponding average current depends on the amount

of ripple current. Lower inductor values (higher

∆IL) will

reduce the load current at which Burst Mode operation

begins.

The output voltage ripple can increase during Burst Mode

operation if

∆IL is substantially less than IBURST. This will

primarily occur when the duty cycle is very close to unity

(VIN is close to VOUT) or if very large value inductors are

chosen. This is generally only a concern in applications

with VOUT ≥ 5V. At high duty cycles, a skipped cycle

causes the inductor current to quickly descend to zero.

However, it takes multiple cycles to ramp the current back

up to IBURST(PEAK). During this interval, the output capaci-

tor must supply the load current and enough charge may

be lost to cause significant droop in the output voltage. It

is a good idea to keep

∆IL comparable to IBURST(PEAK).

Otherwise, one might need to increase the output capaci-

tance in order to reduce the voltage ripple or else disable

Burst Mode operation by forcing continuous operation

with the FCB pin.

Fault Conditions: Current Limit and Output Shorts

The LTC1775 current comparator can accommodate a

maximum sense voltage of 300mV. This voltage and the

sense resistance determine the maximum allowed peak

inductor current. The corresponding output current limit

is:

LIMIT

DS ON

=()()

300

()

–

∆

The current limit value should be checked to ensure that

ILIMIT(MIN) > IO(MAX). The minimum value of current limit

generally occurs with the largest VIN at the highest ambi-

ent temperature, conditions which cause the highest power

dissipation in the top MOSFET. Note that it is important to

check for self-consistency between the assumed junction

temperature of the top MOSFET and the resulting value of

ILIMIT which heats the junction.

Caution should be used when setting the current limit

based upon RDS(ON) of the MOSFETs. The maximum

current limit is determined by the minimum MOSFET on-

resistance. Data sheets typically specify nominal and

maximum values for RDS(ON), but not a minimum. A

reasonable, but perhaps overly conservative, assumption

is that the minimum RDS(ON) lies the same amount below

the typical value as the maximum RDS(ON) lies above it.

Consult the MOSFET manufacturer for further guidelines.

The LTC1775 includes current foldback to help further

limit load current when the output is shorted to ground. If

the output falls by more than half, then the maximum

sense voltage is progressively lowered from 300mV to

about 80mV. Under short-circuit conditions with very low

duty cycle, the LTC1775 will begin skipping cycles in order

to limit the short-circuit current. In this situation the

bottom MOSFET RDS(ON) will control the inductor current

valley rather than the top MOSFET controlling the inductor

current peak. The short-circuit ripple current is deter-

mined by the minimum on-time tON(MIN) of the LTC1775

(approximately 0.5

µs), the input voltage, and inductor

value:

∆IL(SC) = tON(MIN) VIN/L.

The resulting short-circuit current is:

DS ON BOT

LSC

= ()() +

()(

)

()

∆

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