LT1158CS View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1158CS' PDF : 20 Pages View PDF
LT1158
APPLICATIONS INFORMATION
high frequency oscillations –– consult manufacturer’s rec-
ommendations. If individual gate decoupling resistors are
used, the gate feedback pins can be connected to any one
of the gates.
Driving multiple MOSFETs in parallel may restrict the
operating frequency at high supply voltages to prevent
over-dissipation in the LT1158 (see Gate Charge and
Driver Dissipation below). When the total gate capaci-
tance exceeds 10,000pF on the top side, the bootstrap
capacitor should be increased proportionally above 0.1µF.
Gate Charge and Driver Dissipation
A useful indicator of the load presented to the driver by a
power MOSFET is the total gate charge QG, which includes
the additional charge required by the gate-to-drain swing.
QG is usually specified for VGS = 10V and VDS = 0.8VDS(MAX).
When the supply current is measured in a switching
application, it will be larger than given by the DC electrical
characteristics because of the additional supply current
associated with sourcing the MOSFET gate charge:
ISUPPLY
=
IDC
+
dQG
dt
TOP
+
dQG
dt BOTTOM
The actual increase in supply current is slightly higher due
to LT1158 switching losses and the fact that the gates are
being charged to more than 10V. Supply current vs.
switching frequency is given in the Typical Performance
Characteristics.
The LT1158 junction temperature can be estimated by
using the equations given in Note 1 of the electrical
characteristics. For example, the LT1158SI is limited to
less than 25mA from a 24V supply:
TJ = 85°C + (25mA × 24V × 110°C/W)
= 151°C exceeds absolute maximum
In order to prevent the maximum junction temperature
from being exceeded, the LT1158 supply current must be
checked with the actual MOSFETs operating at the maxi-
mum switching frequency.
MOSFET Gate Drive Protection
For supply voltages of over 8V, the LT1158 will protect
standard N-channel MOSFETs from under or overvoltage
gate drive conditions for any input duty cycle including DC.
Gate-to-source zener clamps are not required and not
recommended since they can reduce operating efficiency.
A discontinuity in tracking between the output pulse width
and input pulse width may be noted as the top side
MOSFET approaches 100% duty cycle. As the input low
signal becomes narrower, it may become shorter than the
time required to recharge the bootstrap capacitor to a safe
voltage for the top side driver. Below this duty cycle the
output pulse width will stop tracking the input until the
input low signal is <100ns, at which point the output will
jump to the DC condition of top MOSFET “on” and bottom
MOSFET “off.”
Low Voltage Operation
The LT1158 can operate from 5V supplies (4.5V min.) and
in 6V battery-powered applications by using logic-level
N-channel power MOSFETs. These MOSFETs have 2V
maximum threshold voltages and guaranteed RDS(ON)
limits at VGS = 4V. The switching speed of the LT1158,
unlike CMOS drivers, does not degrade at low supply
voltages. For operation down to 4.5V, the boost pin should
be connected as shown in Figure 2 to maximize gate drive
to the top side MOSFET. Supply voltages over 10V should
not be used with logic-level MOSFETs because of their
lower maximum gate-to-source voltage rating.
N.C.
BOOST DR
BOOST
5V
+
D1
T GATE DR
LT1158
T GATE FB
0.1µF
LOGIC-LEVEL
MOSFET
T SOURCE
D1: LOW-LEAKAGE SCHOTTKY
BAT85 OR EQUIVALENT
LT1158 F02
Figure 2. Low Voltage Operation
9
Share Link: 
