LTC1148CS datasheet(13/20 Pages) LINER

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LTC1148

LTC1148-3.3/LTC1148-5

APPLICATIO S I FOR ATIO

To prevent stray pickup a 100pF capacitor is suggested

across R1 located close to the LTC1148.

For Figure 1 applications with VOUT below 2V, or when

RSENSE is moved to ground, the current sense comparator

inputs operate near ground. When the current comparator

is operated at less than 2V common mode, the off time

increases approximately 40%, requiring the use of a

smaller timing capacitor CT.

Auxiliary Windings – Suppressing Burst Mode

Operation

The LTC1148 synchronous switch removes the normal

limitation that power must be drawn from the inductor

primary winding in order to extract power from auxil-

iary windings. With synchronous switching, auxiliary

outputs may be loaded without regard to the primary

output load, providing that the loop remains in continu-

ous mode operation.

Burst Mode operation can be suppressed at low output

currents with a simple external network which cancels the

25mV minimum current comparator threshold. This tech-

nique is also useful for eliminating audible noise from

certain types of inductors in high current (IOUT > 5A)

applications when they are lightly loaded.

An external offset is put in series with the SENSE – pin to

subtract from the built-in 25mV offset. An example of this

technique is shown in Figure 6. Two 100

Ω resistors are

inserted in series with the leads from the sense resistor.

and

δP = δN = 0.007(63 – 25) = 0.27. The required RDS(ON)

for each MOSFET can now be calculated:

P-Ch RDS(ON) =

12(0.25)

5(2)2 (1.27)

= 0.12

Ω

N-Ch RDS(ON) =

12(0.25)

7(2)2 (1.27)

= 0.085

Ω

The P-channel requirement can be met by a Si9430DY,

while the N-channel requirement is exceeded by a

Si9410DY. Note that the most stringent requirement for

the N-channel MOSFET is with VOUT = 0 (i.e., short circuit).

During a continuous short circuit, the worst-case

N-channel dissipation rises to:

PN = ISC(AVG)2(RDS(ON))(1 + δN)

With the 0.05

Ω sense resistor ISC(AVG) = 2A will result,

increasing the 0.085

ΩN-channeldissipationto450mWat

a die temperature of 73

°C.

CIN will require an RMS current rating of at least 1A at

temperature, and COUT will require an ESR of 0.05Ω for

optimum efficiency.

Now allow VIN to drop to its minimum value. At lower input

voltages the operating frequency will decrease and the

P-channel will be conducting most of the time, causing its

power dissipation to increase. At VIN(MIN) = 7V:

fMIN = (1/2.92µs)[1 – (5V/7V)] = 98kHz

PP =

5V(0.12

Ω)(2A)2(1.27)

= 435mW

This last step is necessary to assure that the power

dissipation and junction temperature of the P-channel are

not exceeded.

LTC1148 Adjustable Applications

When an output voltage other than 3.3V or 5V is required,

the LTC1148 adjustable version is used with an external

resistive divider from VOUT to VFB Pin 9 (see Figure 9). The

regulated voltage is determined by:

VOUT = 1.25

)

)1+ R2

Figure 6. Suppression of Burst Mode Operation

RSENSE

1000pF

100

Ω

100

Ω

COUT

VOUT

SENSE+ (PIN 8)

SENSE – (PIN 7)

LTC1148 • F06

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