LTC1149 查看數據表（PDF） - Linear Technology

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LTC1149

High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1149

LTC1149-3.3/LTC1149-5

UU W U

APPLICATIO S I FOR ATIO

The LTC1149 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.

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0

0

1

2

3

4

5

MAXIMUM OUTPUT CURRENT (A)

1149 F02

Figure 2. RSENSE vs Maximum Output Current

L and CT Selection for Operating Frequency

The LTC1149 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 propor-

tional 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

=

(7.8)(10

f

– 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:

) f = 1 1 – VOUT

tOFF

VIN

where:

) tOFF = (1.3)(104)(CT)

VREG

VOUT

VREG is the desired output voltage (i.e., 5V, 3.3V), while

VOUT is the actual output voltage. Thus VREG/VOUT = 1

when in regulation.

Note that as VIN decreases, the frequency decreases.

When the input to output voltage differential drops below

1.5V, the LTC1149 series reduces tOFF by increasing the

discharge current in CT. This prevents audible operation

prior to dropout.

Once the frequency has been set by CT, the inductor L must

be chosen to provide no more than 25mV/RSENSE of peak-

to-peak inductor ripple current. This results in a minimum

required inductor value of:

LMIN =( 5.1)(105)(RSENSE)(CT)(VREG)

As the inductor value is increased from the minimum

value, the ESR requirements for the output capacitor are

eased at the expense of efficiency. If too small an inductor

is used, the inductor current will decrease past zero and

change polarity. A consequence of this is that the LTC1149

series may not enter Burst Mode operation and efficiency

will be severely degraded at low currents.

1400

1200

VOUT = 5V

1000

800

600

VIN = 48V

400

VIN = 24V

200

0

0

VIN = 12V

50 100 150 200 250

FREQUENCY (kHz)

1149 F03

Figure 3. Timing Capacitor Selection

8

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