8
LT1370
sn1370 1370fs
Shutdown and Synchronization
The device has a dual function S/S pin which is used for
both shutdown and synchronization. This pin is logic level
compatible and can be pulled high, tied to V
IN
or left
floating for normal operation. A logic low on the S/S pin
activates shutdown, reducing the part’s supply current to
12µA. Typical synchronization range is from 1.05 to 1.8
times the part’s natural switching frequency, but is only
guaranteed between 600kHz and 800kHz. A 12µs resetable
shutdown delay network guarantees the part will not go
into shutdown while receiving a synchronization signal.
Caution should be used when synchronizing above 700kHz
because at higher sync frequencies the amplitude of the
internal slope compensation used to prevent subhar-
monic switching is reduced. This type of subharmonic
switching only occurs when the duty cycle of the switch
is above 50%. Higher inductor values will tend to elimi-
nate this problem.
Thermal Considerations
Care should be taken to ensure that the worst-case input
voltage and load current conditions do not cause exces-
sive die temperatures. Typical thermal resistance is
30°C/W for the R package and 50°C/W for the T7 package
but these numbers will vary depending on the mounting
techniques (copper area, airflow, etc.). Heat is transferred
from the package via the tab.
Average supply current (including driver current) is:
I
IN
= 4.5mA + DC(I
SW
/45)
I
SW
= Switch current
DC = Switch duty cycle
Switch power dissipation is given by:
P
SW
= (I
SW
)
2
(R
SW
)(DC)
R
SW
= Output switch ON resistance
Total power dissipation of the die is the sum of supply
current times supply voltage, plus switch power:
P
D(TOTAL)
= (I
IN
)(V
IN
) + P
SW
Surface mount heat sinks are available which can lower
package thermal resistance by two or three times. One
manufacturer, Wakefield Engineering, offers surface mount
heat sinks for the R package and can be reached at (617)
245-5900 or at www.wakefield.com.
Choosing the Inductor
For most applications the inductor will fall in the range of
2.2µH to 22µH. Lower values are chosen to reduce physi-
cal size of the inductor. Higher values allow more output
current because they reduce peak current seen by the
power switch, which has a 6A limit. Higher values also
reduce input ripple voltage and reduce core loss.
When choosing an inductor you need to consider maxi-
mum load current, core and copper losses, allowable
component height, output voltage ripple, EMI, fault
current in the inductor, saturation and, of course, cost.
The following procedure is suggested as a way of handling
these somewhat complicated and conflicting requirements.
1. Assume that the average inductor current for a boost
converter is equal to load current times V
OUT
/V
IN
and
decide whether or not the inductor must withstand
continuous overload conditions. If average inductor
current at maximum load current is 3A, for instance, a
3A inductor may not survive a continuous 6A overload
condition. Also be aware that boost converters are not
short-circuit protected and that, under output short
conditions, inductor current is limited only by the
available current of the input supply.
2. Calculate peak inductor current at full load current to
ensure that the inductor will not saturate. Peak current
can be significantly higher than output current, espe-
cially with smaller inductors and lighter loads, so don’t
omit this step. Powdered iron cores are forgiving
because they saturate softly, whereas ferrite cores
saturate abruptly and other core materials fall in
between. The following formula assumes continuous
mode operation but it errs only slightly on the high side
for discontinuous mode, so it can be used for all
conditions.
APPLICATIO S I FOR ATIO
UU W U
