LTC1622CS8 datasheet(6/16 Pages) LINER

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LTC1622

the ITH pin will drop. When the ITH voltage goes below

0.12V, the sleep signal goes high, turning off the external

MOSFET. The sleep signal goes low when the ITH voltage

rises above 0.22V and the LTC1622 resumes normal

operation. The next oscillator cycle will turn the external

MOSFET on and the switching cycle repeats.

Frequency Synchronization

The LTC1622 can be externally driven by a TTL/CMOS

compatible clock signal up to 750kHz.

Do not synchronize

the LTC1622 below its maximum default operating fre-

quency of 625kHz as this may cause abnormal operation

and an undesired frequency spectrum. The LTC1622 is

synchronized to the rising edge of the clock. The external

clock pulse width must be at least 100ns and not more

than the period minus 200ns.

Synchronization is inhibited when the feedback voltage is

below 0.3V. This is to prevent inductor current buildup

under short-circuit conditions. Burst Mode operation is

deactivated when the LTC1622 is externally driven by a

clock.

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 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 EA. Further reduction 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 LTC1622. When the input supply voltage

drops below 2V, the P-channel MOSFET and all circuitry is

turned off except the undervoltage block, which draws

only several microamperes.

OPERATIO

(Refer to Functional Diagram)

Short-Circuit Protection

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

oscillator will be reduced to about 110kHz. This lower

frequency allows the inductor current to safely discharge,

thereby preventing current runaway. The oscillator’s fre-

quency will gradually increase to its nominal value when

the feedback voltage increases above 0.65V. Note that

synchronization is inhibited until the feedback voltage

goes above 0.3V.

Overvoltage Protection

As a further protection, the overvoltage comparator in the

LTC1622 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 35mV.

Slope Compensation and Peak Inductor Current

The inductor’s peak current is determined by:

ITH

SENSE

()

when the LTC1622 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.

DUTY CYCLE (%)

110

100

1622 F02

70 80 90 100

20 30

40 50

IRIPPLE = 0.4IPK

AT 5% DUTY CYCLE

IRIPPLE = 0.2IPK

AT 5% DUTY CYCLE

VIN = 4.2V

UNSYNC

Figure 2. Maximum Output Current vs Duty Cycle

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