SC417_09 datasheet(16/30 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

When a large capacitor is placed in parallel with R1 (C

TOP

)

OUT

is shown by the following equation.

TOP

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 33% of the voltage at V5V. The switch-

ing regulator turns on with power-save disabled and all

switching is in forced continuous mode.

When EN/PSV is high (above 44% of the voltage at V5V)

for SC417, the switching 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 44% of the voltage at V5V)

for SC427, the switching 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 V

Applications Information (continued)

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