LTC1159CS datasheet(7/20 Pages) LINER

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LTC1159

LTC1159-3.3/LTC1159-5

increases, the output voltage decreases slightly. This causes

the output of the gain stage to increase the current com-

parator threshold, thus tracking the load current.

The sequence of events for Burst Mode operation is very

similar to continuous operation with the cycle interrupted

by the voltage comparator. When the output voltage is at or

above the desired regulated value, the P-channel MOSFET

is held off by comparator V and the timing capacitor con-

tinues to discharge below VTH1. When the timing capacitor

discharges past VTH2, voltage comparator S trips, causing

the internal SLEEP line to go low and the N-channel MOSFET

to turn off.

The circuit now enters sleep mode with both power

MOSFETs turned off. In sleep mode, much of the circuitry

(Refer to Functional Diagram)

OPERATIO

is turned off, dropping the supply current from several

milliamps (with the MOSFETs switching) to 300

µA. When

the output capacitor has discharged by the amount of

hysteresis in comparator V, the P-channel MOSFET is

again turned on and this process repeats. To avoid the

operation of the current loop interfering with Burst Mode

operation, a built-in offset is incorporated in the gain stage.

To prevent both the external MOSFETs from being turned

on at the same time, feedback is incorporated to sense the

state of the driver output pins. Before the N-gate output can

go high, the P-drive output must also be high. Likewise, the

P-drive output is prevented from going low when the

N-gate output is high.

APPLICATIO S I FOR ATIO

The LTC1159 Compared to the LTC1148/LTC1149

Families

The LTC1159 family is closest in operation to the LTC1149

and shares much of the applications information. In addi-

tion to reduced quiescent and shutdown currents, the

LTC1159 adds an internal switch which allows the driver

and control sections to be powered from an external

source for higher efficiency. This change affects Power

MOSFET Selection, EXTVCC Pin Connection, Important

Information About LTC1159 Adjustable Applications, and

Efficiency Considerations found in this section.

The basic LTC1159 application circuit shown in Figure 1

is limited to a maximum input voltage of 30V due to

MOSFET breakdown. If the application does not require

greater than 18V operation, then the LTC1148 or

LTC1148HV should be used. For higher input voltages

where quiescent and shutdown current are not critical, the

LTC1149 may be a better choice since it is set up to drive

standard threshold MOSFETs.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current. The

LTC1159 current comparator has a threshold range that

extends from a minimum of 0.025V/RSENSE to a maximum

of 0.15V/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 opera-

tion, 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) = 0.025V/RSENSE (see CT and L Selection for

Operating Frequency). Solving for RSENSE and allowing

a margin for variations in the LTC1159 and external

component values yields:

RSENSE =

Ω

100

IMAX

A graph for selecting RSENSE versus maximum output

current is given in Figure 2. The LTC1159 series works well

with values of RSENSE from 0.02Ω to 0.2Ω.

The load current below which Burst Mode operation com-

mences, IBURST, and the peak short-circuit current, ISC(PK),

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

ISC(PK) can be predicted from the following equations:

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