LT1910
10
1910fc
For more information www.linear.com/LT1910
APPLICATIONS INFORMATION
Low Voltage/Wide Supply Range Operation
When the supply is less than 12V, the LT1910’s charge
pump does not produce sufficient gate voltage to fully
enhance the standard N-channel MOSFET. For these ap-
plications, a logic-level MOSFET can be used to extend
the operating supply down to 8V. If the MOSFET has a
maximum V
GS
rating of 15V or greater, then the LT1910
can also operate up to a supply voltage of 60V (absolute
maximum rating of the V
+
pin).
Protecting Against Supply Transients
The LT1910 is 100% tested and guaranteed to be safe
from damage with 60V applied between the V
+
and GND
pins. However, when this voltage is exceeded, even for a
few microseconds, the result can be catastrophic. For this
reason it is imperative that the LT1910 is not exposed to
supply transients above 60V. A transient suppressor, such
as Diodes Inc.’s SMAJ48A, should be added between the
V
+
and GND pins for such applications.
For proper current sense operation, the V
+
pin is required
to be connected to the positive side of the drain-sense
resistor (see Drain-Sense Configuration). Therefore, the
supply should be adequately decoupled at the node where
the V
+
pin and drain sense resistor meet. Several hundred
microfarads may be required when operating with a high
current switch.
When the operating voltage approaches the 60V absolute
maximum rating of the LT1910, local supply decoupling
between the V
+
and GND pins is highly recommended. An
RC snubber with a transient suppressor are an absolute
necessity. Note however that resistance should not be
added in series with the V
+
pin because it will cause an
error in the current-sense threshold.
Low Side Driving
Although the LT1910 is primarily targeted at high side
(grounded load) switch applications, it can also be used
for low side (supply connected load) switch applications.
Figures 8a and 8b illustrate the LT1910 driving low side
power MOSFETs. Because the LT1910 charge pump tries
to pump the gate of the N-channel MOSFET above the
supply, a clamp Zener is required to prevent the V
GS
(ab-
solute maximum) of the MOSFET from being exceeded.
The LT1910 gate drive is current limited for this purpose
so that no resistance is needed between the GATE pin
and Zener.
Current sensing for protecting low side drivers can be done
in several ways. In the Figure 8a circuit, the supply voltage
for the load is assumed to be within the supply operating
range of the LT1910. This allows the load to be returned
to supply through current-sense resistor, R
S
, providing
normal operation of the LT1910 protection circuitry.
If the load cannot be returned to supply through R
S
, or
the load supply voltage is higher than the LT1910 supply,
the current sense must be moved to the source of the
low side MOSFET.
Figure 8b shows an approach to source sensing. An
operational amplifier (must common mode to ground) is
used to level shift the voltage across R
S
up to the drain-
sense pin. This approach allows the use of a small sense
resistor which could be made from PC trace material. The
LT1910 restart timer functions the same as in the high side
switch application.
