SC4608 datasheet(9/19 Pages) SEMTECH

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SC4608

POWER MANAGEMENT

Application Information (Cont.)

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 SC4608 is

controlled by an external capacitor. SS, the start-up time

is defined as:

•

where, C is the value of the external capacitor in nF, and

SS is the start-up time in ms.

Over Current Protection

The SC4608 detects over current conditions by sensing

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

MOSFET. The SC4608 determines the high side MOS-

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

tion voltage across the high side MOSFET via the V

in (see

the Typical Application Circuit on page 1) and PHASE pin

during the high side MOSFETs 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 SC4608 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:

)

(

MAX

SET

⋅

The R

DS(ON) sensing used in the SC4608 has an addi-

tional feature that enhances the performance of the over

current protection. Because the R

DS(ON) has a positive

temperature coefficient, the 50µA current source has a

positive coefficient of about 0.3%/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 SC4608 due to the compact layout of

the power supply.

When the converter detects an over current condition

(I > I

MAX) as shown in Figure 2, the first action the SC4608

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 SC4608 is invoked

and the internal soft start capacitor is discharged. This is

like a typical soft start cycle:

Figure 2. Over current protection characteristic of

SC4608

nom

V −

⋅

nom

V −

MAX

nom

V −

⋅

nom

V −

MAX

nom

V −

⋅

nom

V −

MAX

nom

V −

⋅

nom

V −

MAX

Power MOSFET Drivers

The SC4608 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 SC4608 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 SC4608. This control scheme guar-

antees avoidance of cross conduction or shoot through

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