SC4602AEVB datasheet(10/18 Pages) SEMTECH

10 / 18 page

 2006 Semtech Corp.

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SC4602A/B

POWER MANAGEMENT

When the converter detects an over current condition (I

> I

MAX) as shown in Figure 1, the first action the SC4602A/

B takes is to enter 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 SC4602A/B is in-

voked and the internal soft start capacitor is discharged.

This is like a typical soft start cycle.

IMA

nom

V −

⋅

6875

nom

V −

nom

V −

⋅

125

Figure 1. Over current protection characteristic of

SC4602A/B

Power MOSFET Drivers

The SC4602A/B has two drivers for external complemen-

tary power MOSFETs. The driver block consists of one

high side P-MOSFET, 4

Ω driver, PDRV, and one low side

Ω, N-MOSFET driver, NDRV, which are optimized for driv-

ing external power MOSFETs in a synchronous buck con-

verter. The output drivers also have gate drive non-over-

lap mechanism that gives a dead time between PDRV

and NDRV transitions to avoid potential shoot through

problems in the external MOSFETs. By using the proper

design and the appropriate MOSFETs, a 6A converter can

be achieved. As shown in Figure 2, t

d1, the delay from

the P-MOSFET off to the N-MOSFET on is adaptive by

detecting the voltage of the phase node. t

d2, the delay

from the N-MOSFET off to the P-MOSFET on is fixed, is

100ns for the SC4602A/B. This control scheme guaran-

tees avoiding the cross conduction or shoot through be-

Applications Information (Cont.)

tween two MOSFETs and minimizes the conduction loss

in the bottom diode for high efficiency applications.

NMOSFET Gate Drive

PMOSFET 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 SC4602A/B appli-

cation, 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.

The inductor value can be determined according to its

operating point and the switching frequency as follows:

OMAX

)

(

⋅

∆

⋅

−

⋅

Where:

s = switching frequency and

∆I = ratio of the peak to peak inductor current to the

maximum output load current.

The peak to peak inductor current is:

OMAX

•

∆

−

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