SC4605IMSTRT datasheet(6/19 Pages) SEMTECH

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SC4605

POWER MANAGEMENT

Applications Information

Enable

Pulling and holding the FS/SYNC pin below 0.2V initial-

izes the SLEEP mode of the SC4605 with its typical SLEEP

mode supply current of 10

µA. During the SLEEP mode,

the high side and low side MOSFETs are turned off and

the internal soft start voltage is held low.

Oscillator

The oscillator uses an external capacitor between FS and

GND to set the oscillation frequency. The ramp wave-

form is a triangle at the PWM frequency with a peak volt-

age of 2V and a valley voltage of 0.5V. The PWM duty

ratio is limited to a maximum of 97%, which allows the

bootstrap capacitor to be charged during each cycle. The

capacitor tolerance adds to the accuracy of the oscilla-

tor frequency. The approximate operating frequency is

determined by the external capacitor connected to the

FS/SYNC pin as shown below:

−

⋅

In its synchronous mode, a low value resistor needs to

be connected between ground and the frequency set-

ting capacitor, CT. Then an external clock connects to

the junction of the resistor and the capacitor to activate

its synchronous mode. The frequency of the clock can

be used up to 700kHz. This external clock signal should

have a duty cycle from 5% to 10% and the peak voltage

at the junction from the clock signal should be about

0.2V.

UVLO

When the FS/SYNC pin is not pulled and held below 0.2V,

the voltage on the Vcc pin determines the operation of

the SC4605. As Vcc increases during start up, the UVLO

block senses Vcc and keeps the high side and low side

MOSFETs off and the internal soft start voltage low until

Vcc reaches 2.8V. If no faults are present, the SC4605

will initiate a soft start when Vcc exceeds 2.8V. A hyster-

esis (150mV) in the UVLO comparator provides noise

immunity during its start up.

Soft Start

The soft start function is required for step down control-

lers to prevent excess inrush current through the DC bus

during start up. Generally this can be done by sourcing a

controlled current into a timing capacitor and then using

the voltage across this capacitor to slowly ramp up the

error amp reference. The closed loop creates narrow

width driver pulses while the output voltage is low and

allows these pulses to increase to their steady state duty

cycle as the output voltage reaches its regulated value.

With this, the inrush current from the input side is con-

trolled. The duration of the soft start in the SC4605 is

controlled by an internal timing circuit which is used dur-

ing start up and over current to set the hiccup time. The

soft start time can be calculated by:

START

SOFT

720

As can be seen here, the soft start time is switching fre-

quency dependant. For example, if fs = 300kHz,

TSOFT_START = 720/300k = 2.4ms. But if fs = 600kHz,

TSOFT_START = 720/600k = 1.2ms.

The SC4605 implements its soft start by ramping up the

error amplifier reference voltage providing a controlled

slew rate of the output voltage, then preventing over-

shoot and limiting inrush current during its start up.

Over Current Protection

Over current protection for the SC4605 is implemented

by detecting the voltage drop of the high side N-MOSFET

during its conduction, also known as high side RDS(ON)

detection. This loss-less detection eliminates the sense

resistor and its loss. The overall efficiency is improved

and the number of components and cost of the con-

verter are reduced. RDS(ON) sensing is by default inac-

curate and is mainly used to protect the power supply

during a fault case. The over current trigger point will

vary from unit to unit as the RDS(ON) of N-MOSFET var-

ies. Even for the same unit, the over current trigger point

will vary as the junction temperature of N-MOSFET var-

ies. The SC4605 provides a built-in 50

µA current source,

which is combined with RSET (connected between VCC

and ISET) to determine the current limit threshold. The

value of RSET can be properly selected according to the

desired current limit point IMAX and the internal 50

µA

pull down current available on the ISET pin based on the

following expression:

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