SC4605IMSTRT datasheet(7/19 Pages) SEMTECH

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SC4605

POWER MANAGEMENT

Applications Information (Cont.)

)

(

MAX

SET

⋅

Kelvin sensing connections should be used at the drain

and source of N-MOSFET. R

SET

needs to be adjusted if

the input of the application changes significantly, say from

3.3V to 5V for the same load and same output voltage.

A 0.1

µA ceramic capacitor paralled to this resistor should

be used to decouple the noise.

The RDS(ON) sensing used in the SC4605 has an addi-

tional feature that enhances the performance of the over

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

temperature coefficient, the 50

µA current source has a

positive coefficient of about 0.17%/C° providing first order

correction for current sensing vs temperature. This com-

pensation depends on the high amount of thermal trans-

ferring that typically exists between the high side N-

MOSFET and the SC4605 due to the compact layout of

the power supply.

When the converter detects an over current condition (I

> IMAX) as shown in Figure 1, the first action the SC4605

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 68.75% of nominal out-

put voltage) occur at the same time, the Hiccup mode

operation (Point C to Point D) of the SC4605 is invoked

and the internal soft start capacitor is discharged. This is

like a typical soft start cycle.

IMAX

nom

V −

⋅

6875

nom

V −

nom

V −

⋅

125

Figure 1. Over current protection characteristic of

SC4605

Power MOSFET Drivers

The SC4605 has two drivers for external power N-

MOSFETs. The driver block consists of one high side N-

MOSFET, 1A driver, DRVH, and one low side 1A, N-MOSFET

driver, DRVL, which are optimized for driving external

power MOSFETs in a synchronous buck converter. The

output drivers also have gate drive non-overlap mecha-

nism that gives 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, a 12A converter can be

achieved. As shown in Figure 2, 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 50ns for the SC4605. This control scheme guar-

antees avoiding the cross conduction or shoot through

between two MOSFETs and minimizes the conduction loss

in the bottom diode for high efficiency applications.

BOTTOM MOSFET Gate Drive

TOP MOSFET Gate Drive

Phase node

Ground

td1

td2

Figure 2. Timing Waveforms for Gate Drives and Phase

Node

Inductor Selection

The factors for selecting the inductor include its cost,

efficiency, size and EMI. For a typical SC4605 applica-

tion, the inductor selection is mainly based on its value,

saturation current and DC resistance. Increasing the in-

ductor value will decrease the ripple level of the output

voltage while the output transient response will be de-

graded. Low value inductors offer small size and fast tran-

sient responses while they cause large ripple currents,

poor efficiencies and more output capacitance to smooth

out the large ripple currents. The inductor should be able

to handle the peak current without saturating and its

copper resistance in the winding should be as low as

possible to minimize its resistive power loss. A good trade-

off among its size, loss and cost is to set the inductor

ripple current to be within 15% to 30% of the maximum

output current.

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