SC417EVB Ver la hoja de datos (PDF) - Semtech Corporation

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Fabricante

SC417EVB

10A Integrated FET Regulator with Programmable LDO

Semtech Corporation 

SC417/SC427

Applications Information (continued)

FB FB Comparator

-

500mV +

Gate

Drives VIN

DH

Q1

VLX L

VOUT

VIN

One-Shot

Timer

ESR

DL

Q2

COUT

+

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

VOUT

FB

Figure 2 — On-Time Generation

This method automatically produces an on-time that is

proportional to V and inversely proportional to V .

OUT

IN

Under steady-state conditions, the switching frequency

can be determined from the on-time by the following

equation.

fSW

VOUT

TON u VIN

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.

RTON

(TON  10ns) uVIN

25pF uVOUT

The maximum R value allowed is shown by the follow-

TON

ing equation.

RTON _ MAX

VIN _ MIN

15PA

V Voltage Selection

OUT

The switcher output voltage is regulated by comparing

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

OUT

internal 500mV reference voltage, see Figure 3.

VOUT

R1

R2

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 is offset by the output ripple according to the

OUT

following equation.

VOUT

0.5

u

¨¨©§1

R1

R2

¸¸¹·



¨§

©

VRIPPLE

2

¸·

¹

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

OUT

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

IN

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

14

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