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LTC1627

Monolithic Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1627

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OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC1627 uses a constant frequency, current mode

step-down architecture. Both the main and synchronous

switches, consisting of top P-channel and bottom

N-channel power MOSFETs, are internal. During normal

operation, the internal top power MOSFET is turned on

each cycle when the oscillator sets the RS latch, and

turned off when the current comparator, ICOMP, resets the

RS latch. The peak inductor current at which ICOMP resets

the RS latch is controlled by the voltage on the ITH pin,

which is the output of error amplifier EA. The VFB pin,

described in the Pin Functions section, allows EA to

receive an output feedback voltage from an external resis-

tive divider. When the load current increases, it causes a

slight decrease in the feedback voltage relative to the 0.8V

reference, which, in turn, causes the ITH voltage to in-

crease until the average inductor current matches the new

load current. While the top MOSFET is off, the bottom

MOSFET is turned on until either the inductor current

starts to reverse as indicated by the current reversal

comparator IRCMP, or the beginning of the next cycle.

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

pin low. Releasing RUN/SS allows an internal 2.25µA

current source to charge soft-start capacitor CSS. When

CSS reaches 0.7V, the main control loop is enabled with the

ITH voltage clamped at approximately 5% of its maximum

value. As CSS continues to charge, ITH is gradually

released, allowing normal operation to resume.

Comparator OVDET guards against transient overshoots

> 7.5% by turning the main switch off and turning the

synchronous switch on. With the synchronous switch

turned on, the output is crowbarred. This may cause a

large amount of current to flow from VIN if the main switch

is damaged, blowing the system fuse.

Burst Mode Operation

The LTC1627 is capable of Burst Mode operation in which

the internal power MOSFETs operate intermittently based

on load demand. To enable Burst Mode operation, simply

allow the SYNC/FCB pin to float or connect it to a logic

high. To disable Burst Mode operation and enable forced

continuous mode, connect the SYNC/FCB pin to GND. In

this mode, the efficiency is lowest at light loads, but

becomes comparable to Burst Mode operation when the

output load exceeds 100mA. The threshold voltage be-

tween Burst Mode operation and forced continuous mode

is 0.8V. This can be used to assist in secondary winding

regulation as described in Auxiliary Winding Control Using

SYNC/FCB Pin in the Applications Information section.

When the converter is in Burst Mode operation, the peak

current of the inductor is set to approximately 200mA,

even though the voltage at the ITH pin indicates a lower

value. The voltage at the ITH pin drops when the inductor’s

average current is greater than the load requirement. As

the ITH voltage drops below 0.12V, the BURST comparator

trips, causing the internal sleep line to go high and turn off

both power MOSFETs.

In sleep mode, both power MOSFETs are held off and the

internal circuitry is partially turned off, reducing the quies-

cent current to 200µA. The load current is now being

supplied from the output capacitor. When the output

voltage drops, causing ITH to rise above 0.22V, the top

MOSFET is again turned on and this process repeats.

Short-Circuit Protection

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

oscillator is reduced to about 35kHz, 1/10 the nominal

frequency. This frequency foldback ensures that the

inductor current has more time to decay, thereby prevent-

ing runaway. The oscillator’s frequency will progressively

increase to 350kHz (or the synchronized frequency) when

VFB rises above 0.3V.

Frequency Synchronization

The LTC1627 can be synchronized with an external

TTL/CMOS compatible clock signal. The frequency range

of this signal must be from 385kHz to 525kHz. Do not

attempt to synchronize the LTC1627 below 385kHz as this

may cause abnormal operation and an undesired fre-

quency spectrum. The top MOSFET turn-on follows the

rising edge of the external source.

When the LTC1627 is clocked by an external source, Burst

Mode operation is disabled; the LTC1627 then operates in

PWM pulse skipping mode. In this mode, when the output

load is very low, current comparator ICOMP remains tripped

for more than one cycle and forces the main switch to stay

off for the same number of cycles. Increasing the output

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