LTC1779 データシートの表示（PDF） - Linear Technology

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LTC1779

250mA Current Mode Step-Down DC/DC Converter in ThinSOT

Linear Technology 

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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 internal P-channel power

MOSFET and keeping it off until the fault is removed.

Burst Mode Operation

The LTC1779 enters Burst Mode operation at low load

currents. In this mode, the peak current of the inductor is

set as if VITH/RUN = 1V (at low duty cycles) even though

the voltage at the ITH/RUN pin is at a lower value. If the

inductor’s average current is greater than the load require-

ment, the voltage at the ITH/RUN pin will drop. When the

ITH/RUN voltage goes below 0.85V, the sleep signal goes

high, turning off the internal MOSFET. The sleep signal

goes low when the ITH/RUN voltage goes above 0.925V

and the LTC1779 resumes normal operation. The next

oscillator cycle will turn the internal MOSFET on and the

switching cycle repeats.

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 internal 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 LTC1779. 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.

LTC1779

Short-Circuit Protection

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

oscillator will be reduced to about 100kHz. 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.

Overvoltage Protection

As a further protection, the overvoltage comparator in the

LTC1779 will turn the internal MOSFET off when the

feedback voltage has risen 7.5% above the reference

voltage of 0.8V. This comparator has a typical hysteresis

of 30mV.

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

( ) IPK

=

M(VITH/RUN – 0.7)

10 RSENSE + 2Ω

when the LTC1779 is operating below 40% duty cycle.

However, once the duty cycle exceeds 40%, slope com-

pensation begins and effectively reduces the peak induc-

tor current. The amount of reduction is given by the curves

in Figure 2.

110

100

90

80

70

60

50

IRIPPLE = 0.4IPK

40

AT 5% DUTY CYCLE

30

IRIPPLE = 0.2IPK

AT 5% DUTY CYCLE

20 VIN = 4.2V

10

0 10 20 30 40 50 60 70 80 90 100

DUTY CYCLE (%)

1779 F02

Figure 2. Maximum Output Current vs Duty Cycle

5

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