LTC1159CS datasheet(7/20 Pages) LINER

7 / 20 page

LTC1159/LTC1159-3.3/LTC1159-5

APPLICATIO S I FOR ATIO

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

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

IBURST ≈

15mV

RSENSE

ISC(PK) =

150mV

RSENSE

The LTC1159 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.

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 which

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),

L and CT Selection for Operating Frequency

The LTC1159 uses a constant off-time architecture with

tOFF determined by an external timing capacitor CT. The

value of CT is calculated from the desired continuous mode

operating frequency, f:

CT =

7.8

× 10–5

)

)1– VOUT

VIN

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 is given by:

MAXIMUM OUTPUT CURRENT (A)

0.12

0.16

0.20

LTC1159 • F02

0.08

0.04

0.10

0.14

0.18

0.06

0.02

Figure 2. RSENSE vs Maximum Output Current

LTC1159CS Datasheet(PDF) 7 Page - Linear Technology

LTC1159CS Datasheet(HTML) 7 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL