LTC1148CS datasheet(8/20 Pages) LINER

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LTC1148

LTC1148-3.3/LTC1148-5

APPLICATIO S I FOR ATIO

The basic LTC1148 series application circuit (fixed

output versions) is shown in Figure 1. External compo-

nent selection is driven by the load requirement, and

begins with the selection of RSENSE. Once RSENSE is

known, CT and L can be chosen. Next, the power

MOSFETs and D1 are selected. Finally, CIN and COUT are

selected and the loop is compensated. The circuit

shown in Figure 1 can be configured for operation up to

an input voltage of 20V. If the application requires

higher input voltage, then the LTC1149 or LTC1159

should be used.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

The LTC1148 series current comparator has a threshold

range which extends from a minimum of 25mV/RSENSE to

a maximum of 150mV/RSENSE. The current comparator

threshold sets the peak of the inductor ripple current,

yielding a maximum output current IMAX equal to the peak

value less half the peak-to-peak ripple current. For proper

Burst Mode operation, IRIPPLE(P-P) must be less than or

equal to the minimum current comparator threshold.

Since efficiency generally increases with ripple current,

the maximum allowable ripple current is assumed, i.e.,

IRIPPLE(P-P) = 25mV/RSENSE (See CT and L Selection for

Operating Frequency). Solving for RSENSE and allowing

a margin for variations in the LTC1148 series and

external component values yields:

RSENSE =

100mV

IMAX

A graph for selecting RSENSE versus maximum output

current is given in Figure 2.

The load current below which Burst Modeoperation com-

mences (IBURST)andthepeakshort-circuitcurrent(ISC(PK))

both track IMAX. Once RSENSE has been chosen, IBURST and

ISC(PK) can be predicted from the following:

IBURST ≈

15mV

RSENSE

ISC(PK) =

150mV

RSENSE

Figure 2. Selecting RSENSE

MAXIMUM OUTPUT CURRENT (A)

0.15

0.20

LTC1148 • F02

0.10

0.05

The LTC1148 series automatically extends tOFF during a

short circuit to allow sufficient time for the inductor

current to decay between switch cycles. The resulting

ripple current causes the average short-circuit current

ISC(AVG) to be reduced to approximately IMAX.

L and CT Selection for Operating Frequency

The LTC1148 series uses a constant off-time architecture

with tOFF determined by an external timing capacitor CT.

Each time the P-channel MOSFET switch turns on, the

voltage on CT is reset to approximately 3.3V. During the off

time, CT is discharged by a current which is proportional

to VOUT. The voltage on CT is analogous to the current in

inductor L, which likewise decays at a rate proportional to

VOUT. Thus the inductor value must track the timing

capacitor value.

The value of CT is calculated from the desired continuous

mode operating frequency, f:

CT =

2.6(104)f

Assumes VIN = 2VOUT, Figure 1 circuit.

A graph for selecting CT versus frequency including the

effects of input voltage is given in Figure 3.

As the operating frequency is increased the gate charge

losses will be higher, reducing efficiency (see Efficiency

Considerations). The complete expression for operating

frequency of the circuit in Figure 1 is given by:

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