SC3303 datasheet(13/26 Pages) SEMTECH

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SC3303

Applications Information (continued)

PWM

One-Shot

Timer

Time = K x (VOUT/VIN)

VIN

Ref.

COUT

VIN

ESR

VLX

Hi-side

and

Lo-side

Gate

Drivers

RTON

VOUT

LPF

VLX_FILTER

Figure 2 — On-Time Generation

This method automatically produces an on-time that is

proportional to V

OUT

and inversely proportional to V

Under steady-state operating conditions in continuous

conduction mode, the switching frequency can be deter-

mined from the on-time by the following equation.

OUT

The SC3303 uses an external resistor to set the on-time

which indirectly sets the frequency. The on-time can be

programmed to provide operating frequencies from

200kHz to 1MHz using a resistor between the TON pin

and ground. The resistor value is

selected by the follow-

ing equation.

400

25pF

OUT

TON

The maximum R

TON

value allowed is shown by the follow-

ing equation.

MIN

MAX

TON

Immediately after the on-time, the DL (the drive

signal for the low side FET) output drives high to

turn on the low-side MOSFET. DL has a minimum

high time of ~320ns, after which DL continues to

stay high until one of the following occurs:

falls below the 750mV reference

The zero cross detector senses that the voltage

on the LX node is below ground. PSAVE is acti-

vated 8 periods after the zero cross is detected.

•

OUT Voltage Selection

The switcher output voltage is regulated by comparing

OUT

as seen through a resistor divider at the FB pin to the

internal 750mV reference voltage, see Figure 3.

To FB pin

VOUT

Figure 3 — Output Voltage Selection

Note that this control method regulates the valley of the

output ripple voltage, not the DC value. The DC output

voltage V

OUT

is offset by the output ripple according to

the following equation.

RIPPLE

OUT

Enable Input

The EN input is used to enable or disable the switching

regulator. When EN is low (grounded), the switching regu-

lator and LDO are disabled and the SC3303 is in its lowest

power state. When disabled, the output power switches

are tri-stated. When EN is higher than 0.8V, the internal

LDO will be activated. The switching regulator remains off

until the voltage at the EN pin exceeds 1.5V.

The EN pin can be used for implementing UVLO for the

input voltage by configuring a voltage divider from VIN to

EN to PGND.

Continuous Mode Operation

The SC3303 operates in CCM (Continuous Conduction

Mode) at larger load currents when the load is greater

than or equal to half of the inductor ripple current (Figure

4). In this mode one of the power MOSFETs is always on,

with no intentional dead time other than to avoid cross-

conduction. This mode of operation results in uniform

frequency.

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