SC4609EVB Ver la hoja de datos (PDF) - Semtech Corporation

Número de pieza

componentes Descripción

Lista de partido

SC4609EVB

Low Input, MHz Operation, High Efficiency Synchronous Buck

Semtech Corporation 

SC4609

POWER MANAGEMENT

Application Information (Cont.)

Over Current Protection

The SC4609 detects over current conditions by sensing

the voltage across the drain-to-source of the high side

MOSFET. The SC4609 determines the high side MOS-

FET current level by sensing the drain-to-source conduc-

tion voltage across the high side MOSFET via the Vin (see

the Typical Application Circuit on page 1) and PHASE pin

during the high side MOSFET’s conduction period. This

voltage value is then compared internally to a user pro-

grammed current limit threshold. Note that user should

place Kelvin sensing connections directly from the high

side MOSFET source to the PHASE pin.

The current limit threshold is programmed by the user

based on the RDS(on) of the high side MOSFET and the

value of the external set resistor RSET (where RSET is

represented by R3 in the applications schematics of this

document). The SC4609 uses an internal current source

to pull a 50µA current from the input voltage to the ISET

pin through external resistor RSET.

The current limit threshold resistor (RSET) value is calcu-

lated using the following equation:

R SET

=

IMAX ⋅ RDS(ON)

50µA

The RDS(ON) sensing used in the SC4609 has an addi-

tional feature that enhances the performance of the over

current protection. Because the RDS(ON) has a positive

temperature coefficient, the 50µA current source has a

positive coefficient of about 0.28%/C° providing first or-

der correction for current sensing vs temperature. This

compensation depends on the high amount of thermal

transferring that typically exists between the high side N-

MOSFET and the SC4609 due to the compact layout of

the power supply.

When the converter detects an over current condition (I

> IMAX) as shown in Figure 2, the first action the SC4609

takes is to enter the cycle by cycle protection mode (Point

B to Point C), which responds to minor over current cases.

Then the output voltage is monitored. If the over current

and low output voltage (set at 70% of nominal output

voltage) occur at the same time, the Hiccup mode op-

eration (Point C to Point D) of the SC4609 is invoked

and the internal soft start capacitor is discharged. This is

like a typical soft start cycle:

A

VO − nom

00.67 ⋅VO −nom

VO

D

IO

B

C

IMAX

Figure 2. Over current protection characteristic of

SC4609

Power MOSFET Drivers

The SC4609 has two drivers which are optimized for driv-

ing external power N-Channel MOSFETs.. The driver block

consists two 1 Amp drivers. DRVH drives the high side

N-MOSFET (main switch), and DRVL drives the low side

N-MOSFET (synchronous rectifier transistor).

The output drivers also have gate drive non-overlap

mechanism that provides a dead time between DRVH

and DRVL transitions to avoid potential shoot through

problems in the external MOSFETs. By using the proper

design and the appropriate MOSFETs, the SC4609 is

capable of driving a converter with up to 12A of output

current. As shown in Figure 3, td1, the delay from the

top MOSFET off to the bottom MOSFET on is adaptive by

detecting the voltage of the phase node. td2, the delay

from the bottom MOSFET off to the top MOSFET on is

fixed, is 40ns for the SC4609. This control scheme guar-

antees avoidance of cross conduction or shoot through

between the upper and lower MOSFETs and also mini-

mizes the conduction loss in the body diode of the bot-

tom MOSFET for high efficiency applications.

 2006 Semtech Corp.

8

www.semtech.com

Share Link: