LTC1174CS8 datasheet(7/20 Pages) LINER

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LTC1174

LTC1174-3.3/LTC1174-5

1174fe

(Refer to Functional Diagram)

The LTC1174 uses a constant off-time architecture to

switch its internal P-channel power MOSFET. The off-time

is set by an internal timing capacitor and the operating

frequency is a function of VIN.

The output voltage is set by an internal resistive divider

(LTC1174-3.3 and LTC1174-5) or an external divider re-

turned to VFB Pin 1 (LTC1174). A voltage comparator A1

compares the divided output voltage to a reference voltage

of 1.25V.

To optimize efficiency, the LTC1174 automatically switches

between continuous and Burst Mode

® operation. The volt-

age comparator is the primary control element when the

device is in Burst Mode operation, while the current com-

parator controls the output voltage in continuous mode.

During the switch“ON” time, switch current flows through

the 0.1

Ω sense resistor. When this current reaches the

threshold of the current comparator A2, its output signal will

change state, setting the flip-flop and turning the switch off.

The timing capacitor, CT, begins to discharge until its

voltage goes below VTH1. Comparator A4 will then trip,

which resets the flip-flop and causes the switch to turn on

again. Also, the timing capacitor is recharged. The inductor

current will again ramp up until the current comparator A2

trips. The cycle then repeats.

When the load is relatively light, the LTC1174 automatically

goes into Burst Mode operation. The current mode loop is

interrupted when the output voltage reaches the desired

regulated value. The hysteretic voltage comparator A1 trips

when VOUT is above the desired output voltage, shutting off

the switch and causing the timing capacitor to discharge.

This capacitor discharges past VTH1 until its voltage drops

below VTH2. Comparator A5 then trips and a sleep signal is

generated.

In sleep mode, the LTC1174 is in standby and the load

current is supplied by the output capacitor. All unused

circuitry is shut off, reducing quiescent current from

0.45mA to 0.13mA. When the output capacitor discharges

by the amount of the hysteresis of the comparator A1, the

P-channel switch turns on again and the process repeats

itself.

Operating Frequency and Inductor

Since the LTC1174 utilizes a constant off-time architecture,

its operating frequency is dependent on the value of VIN. The

frequency of operation can be expressed as:

OFF

OUT

−

⎛

⎝⎜

⎞

⎠⎟ ()

where tOFF =4µsandVDisthevoltagedropacrossthediode.

Note that the operating frequency is a function of the input

and ouput voltage.

Although the size of the inductor does not affect the fre-

quency, it does affect the ripple current. The peak-to-peak

ripple current is given by:

RIPPLE

OUT

⎛

⎝⎜

⎞

⎠⎟ ()

−

410 6

•

By choosing a smaller inductor, a low ESR output filter

capacitor has to be used (see CIN and COUT). Moreover, core

loss will also increase (see Inductor Core Selection section)

due to higher ripple current.

OPERATIO

Burst Mode is a registered trademark of Linear Technology Corporation.

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