SC508 Просмотр технического описания (PDF) - Semtech Corporation

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SC508

EcoSpeed® DC-DC Buck Controller with Integrated LDO

Semtech Corporation 

SC508/SC508A

Applications Information (continued)

VOUT drifts up to due to leakage

current flowing into COUT

Smart Power Save

Threshold

FB

threshold

DH and DL off

VOUT discharges via inductor

and low-side MOSFET

Normal VOUT ripple

High-side

Drive (DH)

Single DH on-time pulse

after DL turn-off

Low-side

Drive (DL)

DL turns on when Smart

PSAVE threshold is reached

DL turns off when FB

threshold is reached

Normal DL pulse after DH

on-time pulse

Figure 8 — Smart Power-Save

SmartDriveTM

For each DH pulse, the DH driver initially turns on the

high-side MOSFET at a slower speed, allowing a softer,

smooth turn-off of the low-side diode. Once the diode is

off and the LX voltage has risen 0.8V above PGND, the

SmartDrive circuit automatically drives the high-side

MOSFET on at a rapid rate. This technique reduces switch-

ing noise while maintaining high efficiency, reducing the

need for snubbers.

Enable Input for Switching Regulator

The EN input is a logic level input. When EN is low

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

power state. When EN is low and VDDA is above the VDDA

UVLO threshold, the output of the switching regulator

soft-discharges into the VOUT pin through an internal

2kΩ resistor. When EN is a logic high (>1V) the switching

regulator is enabled.

Note that the LDO enable pin (ENL) can also disable the

switching regulator through the VIN UVLO function. Refer

to the ENL Pin and VIN UVLO section.

Current Limit Protection

The SC508 features programmable current limiting, which

is accomplished using the RDS(ON) of the lower MOSFET for

current sensing. The current limit is set by RLIM resistor

which connects from the ILIM pin to the drain of the low-

side MOSFET. When the low-side MOSFET is on, an internal

10μA current flows from the ILIM pin and through the RLIM

resistor, creating a voltage drop across the resistor. While

the low-side MOSFET is on, the inductor current flows

through it and creates a voltage across the RDS(ON). The

voltage across the MOSFET is negative with respect to

PGND. If this MOSFET voltage drop exceeds the voltage

across RLIM, the voltage at the ILIM pin will be negative and

current limit will activate. The current limit then keeps the

low-side MOSFET on and prevents another high-side on-

time, until the current in the low-side MOSFET reduces

enough to bring the ILIM pin voltage up to zero. This

method regulates the inductor valley current at the level

shown by ILIM in Figure 9.

IPEAK

ILOAD

ILIM

Time

Figure 9 — Valley Current Limit

The EN input has internal resistors — 2MΩ pullup to

VDDA, and a 1MΩ pulldown to AGND. These resistors will

normally cause the EN voltage to be near the logic high

trip point as VDDA reaches the VDDA UVLO threshold.

To prevent undesired toggling of EN and erratic start-up

performance, the EN pin should not be allowed to float as

open-circuit.

17

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