LTC3121 datasheet(10/26 Pages) LINER

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LTC3121

3121fa

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operaTion

Internal Current Limit

The current limit comparator shuts off the N-channel

MOSFET switch once its threshold is reached. Peak switch

current is limited to 1.8A, independent of input or output

voltage, except when VOUT is below 1.5V, resulting in the

current limit being approximately half of the nominal peak.

Lossless current sensing converts the peak current sig-

nal of the N-channel MOSFET switch into a voltage that

is summed with the internal slope compensation. The

summed signal is compared to the error amplifier output

to provide a peak current control command for the PWM.

Zero Current Comparator

Thezerocurrentcomparatormonitorstheinductorcurrent

being delivered to the output and shuts off the synchro-

nous rectifier when this current reduces to approximately

50mA. This prevents the inductor current from reversing

in polarity, improving efficiency at light loads.

Oscillator

Theinternaloscillatorisprogrammedtothedesiredswitch-

ing frequency with an external resistor from the RT pin to

GND according to the following formula:

ƒOSC(MHz)=

57.6

RT(kΩ)

⎛

⎝

⎜

⎞

⎠

⎟

The oscillator also can be synchronized to an external

frequency by applying a pulse train to the PWM/SYNC pin.

An external resistor must be connected between RT and

GNDtoprogramtheoscillatortoafrequencyapproximately

25% below that of the externally applied pulse train used

for synchronization. RT is selected in this case according

to this formula:

RT(kΩ) =

73.2

ƒSYNC(MHz)

⎛

⎝

⎜

⎞

⎠

⎟

Output Disconnect

The LTC3121’s output disconnect feature eliminates body

diode conduction of the internal P-channel MOSFET

rectifier. This allows for VOUT to discharge to 0V during

shutdown, and draw no current from the input source. It

also allows for inrush current limiting at turn-on, minimiz-

ing surge currents seen by the input supply. Note that to

obtain the advantages of output disconnect, there must

not be an external Schottky diode connected between SW

and VOUT. The output disconnect feature also allows VOUT

to be pulled high, without reverse current being backfed

into the power source connected to VIN.

Shutdown

The boost converter is disabled by pulling SD below 0.25V

and enabled by pulling SD above 1.6V. Note that SD can

be driven above VIN or VOUT, as long as it is limited to less

than the absolute maximum rating.

Thermal Shutdown

If the die temperature exceeds 170°C typical, the LTC3121

will go into thermal shutdown (TSD). All switches will be

turnedoffuntilthedietemperaturedropsbyapproximately

7°C, when the device re-initiates a soft-start and switching

can resume.

Boost Anti-Ringing Control

When VOUT ≥ VIN + 2V, the anti-ringing control connects

a resistor across the inductor to damp high frequency

ringing on the SW pin during discontinuous current mode

operation when the inductor current has dropped to near

zero. Although the ringing of the resonant circuit formed

by L and CSW (capacitance on SW pin) is low energy, it

can cause EMI radiation.

VCC Regulator

An internal low dropout regulator generates the 4.25V

(nominal) VCC rail from VIN or VOUT, depending upon

operating conditions. VCC is supplied from VIN when VIN

is greater than 3.5V, otherwise the greater of VIN and VOUT

is used. The VCC rail powers the internal control circuitry

and power MOSFET gate drivers of the LTC3121. The VCC

regulator is disabled in shutdown to reduce quiescent

current and is enabled by forcing the SD pin above its

threshold. A 4.7µF or larger capacitor must be connected

between VCC and SGND.

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