LTC3121 datasheet(7/26 Pages) LINER

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LTC3121

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pin FuncTions

SW (Pin 1): Switch Pin. Connect an inductor from this

pin to VIN. Keep PCB trace lengths as short and wide as

possible to reduce EMI and voltage overshoot. When VOUT

≥ VIN + 2V, an internal anti-ringing resistor is connected

betweenSWandVINaftertheinductorcurrenthasdropped

to near zero, to minimize EMI. The anti-ringing resistor is

also activated in shutdown and during the sleep periods

of Burst Mode operation.

PGND (Pin 2, Exposed Pad Pin 13): Power Ground. When

laying out your PCB, provide a short, direct path between

PGNDandthe outputcapacitorandtiedirectlytotheground

plane. The exposed pad is ground and must be soldered

to the PCB ground plane for rated thermal performance.

VIN (Pin 3): Input Supply Pin. The device is powered from

VIN unless VOUT exceeds VIN and VIN is less than 3V. Place

a low ESR ceramic bypass capacitor of at least 4.7µF from

VIN to PGND. X5R and X7R dielectrics are preferred for

their superior voltage and temperature characteristics.

PWM/SYNC (Pin 4): Burst Mode Operation Select and

Oscillator Synchronization. Do not leave this pin floating.

• PWM/SYNC = High. Disable Burst Mode Operation and

maintain low noise, constant frequency operation.

• PWM/SYNC = Low. The converter operates in Burst

Mode operation, independent of load current.

• PWM/SYNC = External CLK. The internal oscillator is

synchronized to the external CLK signal. Burst Mode

operation is disabled. A clock pulse width between

100ns and 2µs is required to synchronize the oscillator.

An external resistor must be connected between RT

and GND to program the oscillator slightly below the

desired synchronization frequency.

In non-synchronized applications, repeated clocking of

the PWM/SYNC pin to affect an operating mode change

is supported with these restrictions:

• Boost Mode (VOUT > VIN): IOUT <500µA: ƒPWM/SYNC ≤

100Hz, IOUT ≥ 500µA: ƒPWM/SYNC ≤ 5kHz

• Buck Mode (VOUT < VIN): IOUT <5mA: ƒPWM/SYNC ≤ 5Hz,

IOUT ≥ 5mA: ƒPWM/SYNC ≤ 5kHz

VCC (Pin 5): VCC Regulator Output. Connect a low-ESR

filter capacitor of at least 4.7µF from this pin to GND to

provide an internal regulated rail approximately equal to

the lower of VIN and 4.25V. When VOUT is higher than VIN,

and VIN falls below 3V, VCC will regulate to the lower of

approximately VOUTand4.25V.AUVLOeventoccursifVCC

drops below 1.6V. Switching is inhibited, and a soft-start

is initiated when VCC returns above 1.7V.

RT (Pin 6): Frequency Adjust Pin. Connect an external

resistor (RT) from this pin to SGND to program the oscil-

lator frequency according to the formula:

RT = 57.6/ƒOSC

where ƒOSC is in MHz and RT is in kΩ.

VC (Pin 7): Error Amplifier Output. A frequency compen-

sation network is connected to this pin to compensate

the control loop. See Compensating the Feedback Loop

section for guidelines.

FB (Pin 8): Feedback Input to the Error Amplifier. Connect

the resistor divider tap to this pin. Connect the top of the

divider to VOUT and the bottom of the divider to SGND.

The output voltage can be adjusted from 2.2V to 15V ac-

cording to this formula:

VOUT = 1.202V • (1 + R1/R2)

SD(Pin9):LogicControlledShutdownInput.Bringingthis

pin above 1.6V enables normal, free-running operation,

forcing this pin below 0.25V shuts the LTC3121 down, with

quiescentcurrentbelow1μA.Donotleavethispinfloating.

SGND (Pin 10): Signal Ground. When laying out a PC

board, provide a short, direct path between SGND and

the (–) side of the output capacitor.

VOUT (Pin 11): Output Voltage Sense and the Source of

the Internal Synchronous Rectifier MOSFET. Driver bias

is derived from VOUT. Connect the output filter capacitor

from VOUT to PGND, as close to the IC as possible. A

minimum value of 10µF ceramic is recommended. VOUT

is disconnected from VIN when SD is low.

CAP (Pin 12): Serves as the Low Reference for the Syn-

chronous Rectifier Gate Drive. Connect a low ESR filter

capacitor (typically 100nF) from this pin to VOUTtoprovide

an elevated ground rail, approximately 5.6V below VOUT,

used to drive the synchronous rectifier.

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