SC32300ASLASHB.EVB datasheet(11/18 Pages) SEMTECH

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SC32300B

FCCM/Diode Emulation Mode Operation

There are two operating modes which can be configured

with the FCCM pin of the SC32300B. When the FCCM pin

is floated or tied to a logic high signal (e.g. VDD), the drive

signal DL is always complementary to DH. This is called

forced continuous conduction mode (FCCM) operation.

If the FCCM pin is grounded, a zero-crossing circuit is

activated in the SC32300B. The DL output is turned to low

when the current in the external low-side MOSFET reaches

zero. In other words, the external low-side MOSFET acts

like a diode. This operation mode is thus called diode

emulation mode. The benefit of this diode emulation

mode is to reduce conduction losses at light load for high

efficiency.

SmartdriveTM

For each DH drive pulse, the SC32300B initially turns on

the high-side MOSFET with a weak driver, allowing a

softer, smooth turnoff of the low-side MOSFET. Once the

low-side MOSFET is turned off and the LX pin voltage has

risen about 2.5V above ground, the SC32300B switches

to a much stronger driver to complete the turn-on of the

high-side MOSFET at a more rapid rate. This technique

reduces the voltage ringing for the switching node, which

relieves the need for a snubber or a gate resistor.

Bootstrap Capacitor

The SC32300B integrates the bootstrap switch. Only an

external bootstrap capacitor connected between the BST

pin and the LX pin is needed for a complete bootstrap

circuit.Thebootstrapcapacitor,whichischargedfromVDD

during the on time of the low-side MOSFET, provides the

energy to turn on the high-side MOSFET. The capacitance

for the bootstrap capacitor is mainly determined by the

gate charge Q

g of the high-side MOSFET. The equation

below gives a first order estimate:

b = Qg / ΔVb

Where ΔV

b is the allowable voltage drop of the drive

voltage for the high-side MOSFET. In a real application,

considering the capacitance tolerance, Q

g variation,

leakage current of the BST pin etc., a capacitor with two

times the calculated value is a good starting point. A

ceramic capacitor of X5R or X7R, or equivalent material,

should be used.

VLDO Decoupling Capacitor

The VLDO pin is the 3.3V LDO output pin. This 3.3V LDO

can be used as the bias for a microcontroller or any other

digital controller which requires quite, accurate 3.3V

bias on the system board. A high frequency decoupling

capacitor with low ESR and low ESL should be put from

the VLDO pin to the GND pin with minimum trace length.

A typical value of 2.2µF, 6.3V rating, X5R or X7R ceramic

capacitor is recommended.

VDD Decoupling Capacitor

The VDD is the input bias for the low-side driver and the

internal control circuitry. This pin is also used for the VDD

UVLO check. A high frequency decoupling capacitor

with low ESR and low ESL should be put from the VDD

pin to the GND pin with minimum trace length. A typical

value of 2.2µF, 10V rating, X5R or X7R ceramic capacitor is

recommended.

Power Dissipation

The power dissipation in this driver consists of two main

components: the power loss for turning on and off the

external MOSFETs, and the power loss in the internal LDO

(if used). The below equation can be used to estimate the

total power dissipation in the driver:

d = VDD fs (Qgh + Qgl) + (VDD - 3.3) IL

Where:

s is the switching frequency

gh is the total gate charge of the high-side MOSFET

gl is the total gate charge of the low-side MOSFET

L is the external load connected to the VLDO pin

To ensure proper operation and long-term reliability,

the power dissipation should be examined in the real

application to make sure the junction temperature will

not go beyond 125OC. Multiple vias should be used to

connect the thermal pad underneath the driver package

to the ground plane on the system board for good thermal

Application Information (Continued)

SC32300A/B.EVB Datenblatt(PDF) 11 Page - Semtech Corporation

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