SC417 datasheet(14/29 Pages) SEMTECH

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SC417/SC427

Gate

Drives

FB Comparator

One-Shot

Timer

On-time = K x RTON x (VOUT/VIN)

VOUT

VIN

500mV

COUT

VIN

ESR

VOUT

VLX

RTON

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 conditions, the switching frequency

can be determined from the on-time by the following

equation.

OUT

The SC417/SC427 uses an external resistor to set the on-

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

be programmed to provide operating frequency from

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

ground. The resistor value is

selected by the following

equation.

OUT

TON

)

TON

(

The maximum R

TON

value allowed is shown by the follow-

ing equation.

MIN

MAX

TON

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 500mV reference voltage, see Figure 3.

VOUT

To FB pin

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 and Power-save Inputs

The EN/PSV and ENL inputs are used to enable or disable

the switching regulator and the LDO. When EN/PSV is low

(grounded), the switching regulator is off and in its lowest

power state. When off, the output of the switching regula-

tor soft-discharges the output into a 15Ω internal resistor

via the V

OUT

pin. When EN/PSV is allowed to float, the pin

voltage will float to 1.5V. The switching regulator turns on

with power-save disabled and all switching is in forced

continuous mode.

When EN/PSV is high (above 2.0V) for SC417, the switch-

ing regulator turns on with ultra-sonic power-save

enabled. The SC417 ultra-sonic power-save operation

maintains a minimum switching frequency of 25kHz, for

applications with stringent audio requirements.

When EN/PSV is high (above 2.0V) for SC427, the switch-

ing regulator turns on with power-save enabled. The

SC427 power-save operation is designed to maximize

efficiency at light loads with no minimum frequency limits.

This makes the SC427 an excellent choice for portable and

battery-operated systems.

The ENL input is used to control the internal LDO. This

input serves a second function by acting as a V

ULVO

sensor for the switching regulator.

When ENL is low (grounded), the LDO is off. When ENL is

a logic high but below the VIN UVLO threshold (2.6V

typical), then the LDO is on and the switcher is off. When

ENL is above the VIN UVLO threshold, the LDO is enabled

and the switcher is also enabled if the EN/PSV pin is not

grounded.

Forced Continuous Mode Operation

The SC417/SC427 operates the switcher in Forced

Continuous Mode (FCM) by floating the EN/PSV pin (see

Figure 4). In this mode one of the power MOSFETs is

Applications Information (continued)

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