8

 2002 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1205H

PRELIMINARY

The top MOS.ET source must be close to the bottom

MOS.ET drain to prevent ringing and the possibility of

the phase node going negative. This frequency is deter-

mined by:

)

Coss

*

L

(

Sqrt

*

2

(

1

Fring

ST

Π

=

-Where:

L

to trace inductance of the connection between top .ET’s

source and the bottom .ET’s drain added to the trace

resistance of the bottom .ET’s ground connection.

Coss = Drain to source capacitance of bottom .ET. If

there is a Schottky used, the capacitance of the Schottky

is added to this value.

Although this ringing does not pose any power losses due

to a fairly high Q, it could cause the phase node to go too

far negative, thus causing improper operation, double

pulsing or at worst driver damage. On the SC1205H,

the drain node, DRN, can go as far as 2V below ground

without affecting operation or sustaining damage.

The ringing is also an EMI nuisance due to its high reso-

nant frequency. Adding a capacitor, typically 1000-

2000pf, in parallel with Coss of the bottom .ET can of-

ten eliminate the EMI issue.

Prevent Driver Overvoltage

The negative voltage spikes on the phase node adds to

the bootstrap capacitor voltage, thus increasing the volt-

age between VBST - VDRN. This is of special importance

if higher boost voltages are used. If the phase node

negative spikes are too large, the voltage on the boost

capacitor could exceed device’s absolute maximum rat-

ing of 12V. To eliminate the effect of the ringing on the

boost capacitor voltage, place a 4.7 - 10 Ohm resistor

between boost Schottky diode and Vcc to filter the nega-

tive spikes on DRN Pin. Alternately, a Silicon diode, such

as the commonly available 1N4148 can substitute for

the Schottky diode and eliminate the need for the series

resistor.

Proper layout will guarantee minimum ringing and elimi-

nate the need for external components. Use of SO-8 or

other surface mount MOS.ETs while increasing thermal

resistance, will reduce lead inductance as well as radi-

ated EMI.

Over Temperature Shutdown

The SC1205H will shutdown by pulling both driver if its

junction temperature, T

Applications Information (Cont.)