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LTC1267-ADJ

Dual High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1267

LTC1267-ADJ/LTC1267-ADJ5

UU W U

APPLICATIO S I FOR ATIO

The LTC1267 Compared to the LTC1159, LTC1149 and

LTC1142 Family

The LTC1267 family is a dual LTC1159. Identical to the

LTC1159, the LTC1267 can reduce the quiescent and

shutdown currents by making use of an internal switch

which allows the driver and control sections to be

powered from an external source to improve efficiency.

The basic LTC1267 application circuit shown in Figure

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

external MOSFET breakdown. If the application does

not require greater than 18V operation the LTC1142HV

should be used.

Component Selection

The basic LTC1267 application circuit is shown in Figure

1. External component 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 diode are selected. Finally, CIN and

COUT are selected and the loop is compensated. Since the

adjustable, 3.3V and 5V sections in the LTC1267 are

identical, the process of component selection is the same

for both sections.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

The LTC1267 current comparators have 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 LTC1267 and external compo-

nent values yields:

RSENSE

=

100mV

IMAX

The LTC1267 works well with values of RSENSE from

0.02Ω to 0.2Ω. Figure 2 shows the selection of RSENSE vs

maximum output current.

0.20

0.15

0.10

0.05

0

0

1

2

3

4

5

MAXIMUM OUTPUT CURRENT (A)

LTC1267 • F02

Figure 2. Selecting RSENSE

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:

IBURST

≈

15mV

RSENSE

ISC(PK) =

150mV

RSENSE

The LTC1267 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.

CT and L Selection for Operating Frequency

Each regulator section of the LTC1267 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 (fO):

) CT =

7.8 × 10–5

fO

1–

VOUT

VIN

A graph for selecting CT vs frequency including the effects

of input voltage is given in Figure 3.

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