6

LTC1874

(Refer to Functional Diagram)

The LTC1874 is a dual, constant frequency current mode

switching regulator. The two switching regulators func-

tion identically but independent of each other. The follow-

ing description of operation is written for a single

switching regulator.

Main Control Loop

During normal operation, the external P-channel power

MOSFET is turned on by the oscillator and turned off when

output of the error amplifier EAMP. An external resistive

relative to the 0.8V reference, which in turn causes the

current matches the new load current.

current source to charge up the external compensation

pulled up to its zero current level of approximately 0.7V.

As the external compensation network continues to charge

up, the corresponding output current trip level follows,

allowing normal operation.

Comparator OVP guards against transient overshoots

greater than 7.5% by turning off the external P-channel

power MOSFET and keeping it off until the fault is

removed.

Burst Mode Operation

The controller 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 controller 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 controller. 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.

Overvoltage Protection

As a further protection, the overvoltage comparator in the

controller 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.

OPERATIO