SC4612HSTRT datasheet(11/21 Pages) SEMTECH

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SC4612H

POWER MANAGEMENT

Applications Information (Cont.)

Under Voltage Lock Out

Under Voltage Lock Out (UVLO) circuitry senses the VDD

through a voltage divider. If this signal falls below 4.5V

(typical) with a 400mV hysteresis (typical), the output driv-

ers are disabled. During the thermal shutdown, the out-

put drivers are disabled.

OVERCURRENT PROTECTION

The SC4612H features low side MOSFET R

DS(ON) current

sensing and hiccup mode over current protection. The

voltage across the bottom FET is sampled approximately

150ns after it is turned on to prevent false tripping due to

ringing of the phase node.

The internally set over current threshold is 100mV typical.

This can be adjusted up or down by connecting a resistor

between ILIM and DRV or GND respectively. When

programming with an external resistor, threshold set point

accuracy will be degraded to 30%. The FET R

DS(ON) at

temperature will typically be 150% or more of the room

temperature value. Allowance should be made for these

sources of error when programming a threshold value.

When an over current event occurs, the SC4612H

immediately disables both gate drives. The SS ramp

continues to its final value, if not already there. Once at

final value, the SS capacitor is discharged at approximately

1uA until SS low value is reached (approx 0.8V). The SS/

Hiccup cycle will then repeat until the fault condition is

removed and the SC4612H starts up normally on the next

SS cycle.

Gate Drive/Control

The SC4612H provides integrated high current drivers for

fast switching of large MOSFETs. The higher gate current

will reduce switching losses of the larger MOSFETs.

The low side gate drive is supplied directly from the DRV.

The high side gate drive is bootstraped from the DRV pin.

Cross conduction prevention circuitry ensures a non over-

lapping (30ns typical) gate drive between the top and bot-

tom MOSFETs. This prevents shoot through losses which

provides higher efficiency. Typical total minimum off time

for the SC4612H is about 30ns.

ERROR AMPLIFIER DESIGN

The SC4612H is a voltage mode buck controller that utilizes

an externally compensated high bandwidth error amplifier

to regulate the output voltage. The power stage of the

synchronous rectified buck converter control-to-output

transfer function is as shown below.













sESR

)

(

where,

IN – Input voltage

L – Output inductance

ESR

C – Output capacitor ESR

S – Peak to peak ramp voltage

L – Load resistance

C – Output capacitance

The classical Type III compensation network can be built

around the error amplifier as shown below:

Vref

Figure 1. Voltage mode buck converter compensation

network. The transfer function of the compensation

network is as follows:

)

)(

(

)

)(

(

)

(

COMP

⋅

where,

Cout

Lout

)

(

)

(

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