LTC1625IGN datasheet(11/24 Pages) LINER

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LTC1625

APPLICATIONS INFORMATION

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 LTC1625 current comparator can accommodate a

maximum sense voltage of 150mV. This voltage and the

sense resistance determine the maximum allowed peak

inductor current. The corresponding output current limit

is:

LIMIT

DS ON

=()( )

150

()

–

∆

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 LTC1625 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 150mV to

30mV. Under short-circuit conditions with very low duty

cycle, the LTC1625 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 trough

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 LTC1625

(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

= ()( )+

()(

)

()

∆

Normally, the top and bottom MOSFETs will be of the same

type. A bottom MOSFET with lower RDS(ON) than the top

may be chosen if the resulting increase in short-circuit

current is tolerable. However, the bottom MOSFET should

never be chosen to have a higher nominal RDS(ON) than the

top MOSFET.

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy

or Kool M

µ® cores. Actual core loss is independent of core

size for a fixed inductor value, but it is very dependent on

the inductance selected. As inductance increases, core

losses go down. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses

will increase.

Kool M

µ is a registered trademark of Magnetics, Inc.

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