SC401B datasheet(22/32 Pages) SEMTECH

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SC401B

Due to the initial current limitations on the LDO during

power up (Figure 10), any external load attached to the

VDD pin must be limited to less than the start up current

before the LDO has reached 90% of its final regulation

value.

VVLDO Final

90% of

VVLDO Final

Constant current

startup @ ~ 115mA

Voltage regulating

with ~ 200mA

current limit

Short-circuit Protection @ ~ 65mA

0.7V

Figure 10 — LDO Start-Up

LDO Switch-Over Operation

The SC401B includes a switch-over function for the LDO.

The switch-over function is designed to increase efficiency

by using the more efficient DC-DC converter to power the

LDO output, avoiding the less efficient LDO regulator

when possible. The switch-over function connects the

VDD pin directly to the VOUT pin using an internal switch.

When the switch-over is complete the LDO is turned off,

which results in a power savings and maximizes efficiency.

If the LDO output is used to bias the SC401B, then after

switch-over the device is self-powered from the switching

regulator with the LDO turned off.

The switch-over starts 32 switching cycles after PGOOD

output goes high. The voltages at the VDD and VOUT pins

are then compared; if the two voltages are within ±300mV

of each other, the VDD pin connects to the VOUT pin using

an internal switch, and the LDO is turned off. To avoid

unwanted switch-over, the minimum difference between

the voltages for VOUT and VDD should be ±500mV.

It is not recommended to use the switch-over feature for

an output voltage less than VDD UVLO threshold since the

SC401B is not operational below that threshold.

Switch-over MOSFET Parasitic Diodes

The switch-over MOSFET contains parasitic diodes that

are inherent to its construction, as shown in Figure 11. If

the voltage at the VOUT pin is higher than VDD, then the

respective diode will turn on and the current will flow

through this diode. This has the potential of damaging

the device. Therefore, V

OUT must be less than VDD to

prevent damaging the device.

Switchover

MOSFET

Parasitic diode

VDD

LDO

VOUT

Switchover

control

Figure 11— Switch-over MOSFET Parasitic Diodes

Design Procedure

When designing a switch mode supply the input voltage

range, load current, switching frequency, and inductor

ripple current must be specified.

The maximum input voltage (V

INMAX) is the highest speci-

fied input voltage. The minimum input voltage ( V

INMIN) is

determined by the lowest input voltage after evaluating

the voltage drops due to connectors, fuses, switches, and

PCB traces.

The following parameters define the design.

Nominal output voltage (V

OUT)

Static or DC output tolerance

Transient response

Maximum load current (I

OUT)

•

Applications Information (continued)

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