SC4607 データシートの表示（PDF） - Semtech Corporation

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SC4607

Very Low Input, MHz Operation,High Efficiency Synchronous Buck

Semtech Corporation 

SC4607

POWER MANAGEMENT

Application Information (Cont.)

slew rate of the output voltage, then preventing over-

shoot and limiting inrush current during its start up. Dur-

ing start up of a converter with a big capacitive load, the

load current demands large supply current. To avoid this

an external soft start scheme can be implemented as

shown in Figure 2. Cs can be adjusted for different appli-

cations.

current protection. Because the R has a positive

DS(ON)

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 SC4607 due to the compact layout of

the power supply.

Vo

Output of a converter

Cs

330n

Rs

2.05k

Q

MMBT2222A-7

Pin COMP

Rp

47.5k

Figure 2

Over Current Protection:

When the converter detects an over current condition (I

> IMAX) as shown in Figure 3, the first action the SC4607

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 SC4607 is invoked

and the internal soft start capacitor is discharged. This is

like a typical soft start cycle:

The SC4607 detects over current conditions by sensing

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

MOSFET. The SC4607 determines the high side MOSFET

current level by sensing the drain-to-source conduction

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

Typical Application Circuit on page 1) and PHASE pin dur-

ing the high side MOSFET’s conduction period. This volt-

age 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 SC4607 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 SC4607 has an addi-

tional feature that enhances the performance of the over

A

VO − nom

0.6 ⋅VO−nom

VO 0.7

D

IO

B

C

IMAX

Figure 3. Over current protection characteristic of

SC4607

Power MOSFET Drivers:

The SC4607 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 SC4607 is

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

 2005 Semtech Corp.

9

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