LTC1772B データシートの表示（PDF） - Linear Technology

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LTC1772B

Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23

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OPERATIO (Refer to Functional Diagram)

up, the corresponding output current trip level follows,

allowing normal operation.

Comparator OVP guards against transient overshoots

> 7.5% by turning off the external P-channel power

MOSFET and keeping it off until the fault is removed.

Low Load Current Operation

Under very light load current conditions, the ITH/RUN pin

voltage will be very close to the zero current level of 0.85V.

As the load current decreases further, an internal offset at

the current comparator input will assure that the current

comparator remains tripped (even at zero load current)

and the regulator will start to skip cycles, as it must, in

order to maintain regulation. This behavior allows the

regulator to maintain constant frequency down to very

light loads, resulting in less low frequency noise genera-

tion over a wide load current range.

Figure 2 illustrates this result for the circuit of Figure 1

using both an LTC1772 in Burst Mode operation and an

LTC1772B (non-Burst Mode operation). At an output

current of 100mA, the Burst Mode operation part exhibits

an output ripple of approximately 60mVP-P, whereas the

non-Burst Mode operation part has an output ripple of

only 20mVP-P. At lower output current levels, the improve-

ment is even greater. This comes at a tradeoff of lower

efficiency for the non-Burst Mode operation part (see

Figure 1). Also notice the constant frequency operation of

the LTC1772B, even at 5% of maximum output current.

LTC1772B

Dropout Operation

When the input supply voltage decreases towards the

output voltage, the rate of change of inductor current

during the ON cycle decreases. This reduction means that

the external P-channel MOSFET will remain on for more

than one oscillator cycle since the inductor current has not

ramped up to the threshold set by EAMP. Further reduc-

tion in input supply voltage will eventually cause the

P-channel MOSFET to be turned on 100%, i.e., DC. The

output voltage will then be determined by the input voltage

minus the voltage drop across the MOSFET, the sense

resistor and the inductor.

Undervoltage Lockout

To prevent operation of the P-channel MOSFET below safe

input voltage levels, an undervoltage lockout is incorpo-

rated into the LTC1772B. When the input supply voltage

drops below approximately 2.0V, the P-channel MOSFET

and all circuitry is turned off except the undervoltage

block, which draws only several microamperes.

Short-Circuit Protection

When the output is shorted to ground, the frequency of the

oscillator will be reduced to about 120kHz. This lower

frequency allows the inductor current to safely discharge,

thereby preventing current runaway. The oscillator’s fre-

quency will gradually increase to its designed rate when

the feedback voltage again approaches 0.8V.

VOUT Ripple for Figure 1 Circuit Using

LTC1772 Burst Mode Operation.

VOUT Ripple for Figure 1 Circuit Using

LTC1772B Non-Burst Mode Operation.

20mVAC/DIV

20mVAC/DIV

VIN = 3.6V

VOUT = 2.5V

IOUT = 100mA

5µs/DIV

1772 F02a

VIN = 3.6V

VOUT = 2.5V

IOUT = 100mA

5µs/DIV

Figure 2. Output Ripple Waveforms for the Circuit of Figure 1.

1772 F02b

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