LT1158
APPLICATIONS INFORMATION
If individual gate decoupling resistors are used, the gate
feedback pins can be connected to any one of the gates.
MOSFET Gate Drive Protection
For supply voltages of over 8V, the LT1158 will protect
standardN-channelMOSFETsfromunderorovervoltage
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.
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
DriverDissipationbelow).Whenthetotalgatecapacitance
exceeds 10,000pF on the top side, the bootstrap capacitor
should be increased proportionally above 0.1μF.
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.”
Gate Charge and Driver Dissipation
A useful indicator of the load presented to the driver by a
power MOSFET is the total gate charge Q , which includes
G
theadditionalchargerequiredbythegate-to-drainswing.Q
G
is usually specified for V = 10V and V = 0.8V .
GS
DS
DS(MAX)
When the supply current is measured in a switching ap-
plication, it will be larger than given by the DC electrical
characteristics because of the additional supply current
associated with sourcing the MOSFET gate charge:
Low Voltage Operation
dQ
dt
dQ
G
dt
⎛
⎞
⎛
⎞
G
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
ISUPPLY =IDC +
+
⎜
⎝
⎟
⎠
⎜
⎝
⎟
⎠
TOP
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.
maximumthresholdvoltagesandguaranteedR
limits
DS(ON)
at V = 4V. The switching speed of the LT1158, unlike
GS
CMOS drivers, does not degrade at low supply voltages.
For operation down to 4.5V, the boost pin should be con-
nected 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.
The LT1158 junction temperature can be estimated by
using the equations given in Note 1 of the electrical char-
acteristics. For example, the LT1158SI is limited to less
than 25mA from a 24V supply:
5V
N.C.
T
= 85°C + (25mA • 24V • 110°C/W)
= 151°C exceeds absolute maximum
J
+
D1
BOOST DR
BOOST
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
maximum switching frequency.
0.1μF
T GATE DR
T GATE FB
T SOURCE
LT1158
LOGIC-LEVEL
MOSFET
D1: LOW-LEAKAGE SCHOTTKY
BAT85 OR EQUIVALENT
LT1158 F02
Figure 2. Low Voltage Operation
1158fb
10
LINEAR_DIMENSIONS [ Linear Dimensions ]
