5

LTC1772

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.

Burst Mode Operation

The LTC1772 enters Burst Mode operation at low load

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

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

high, turning off the external MOSFET. The sleep signal

and the LTC1772 resumes normal operation. The next

oscillator cycle will turn the external 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 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 LTC1772. 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.

Figure 2. Maximum Output Current vs Duty Cycle

DUTY CYCLE (%)

110

100

90

80

70

60

50

40

30

20

10

1772 F02

0

70 80 90 100

60

10

20 30

40 50

AT 5% DUTY CYCLE

AT 5% DUTY CYCLE

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.

Overvoltage Protection

As a further protection, the overvoltage comparator in the

LTC1772 will turn the external MOSFET off when the

feedback voltage has risen 7.5% above the reference

voltage of 0.8V. This comparator has a typical hysteresis

of 20mV.

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

I

V

R

PK

ITH

SENSE

=

()

10

when the LTC1772 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.