LTC1772HS6 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1772HS6

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

Linear Technology 

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OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC1772 is a constant frequency current mode switch-

ing regulator. During normal operation, the external

P-channel power MOSFET is turned on each cycle when

the oscillator sets the RS latch (RS1) and turned off when

the current comparator (ICMP) resets the latch. The peak

inductor current at which ICMP resets the RS latch is

controlled by the voltage on the ITH/RUN pin, which is the

output of the error amplifier EAMP. An external resistive

divider connected between VOUT and ground allows the

EAMP to receive an output feedback voltage VFB. When the

load current increases, it causes a slight decrease in VFB

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

ITH/RUN voltage to increase until the average inductor

current matches the new load current.

The main control loop is shut down by pulling the ITH/RUN

pin low. Releasing ITH/RUN allows an internal 0.5µA

current source to charge up the external compensation

network. When the ITH/RUN pin reaches 0.35V, the main

control loop is enabled with the ITH/RUN voltage then

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

> 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

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 external MOSFET. The sleep signal

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

and the LTC1772 resumes normal operation. The next

LTC1772

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.

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.

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