LTC1693
U
W U U
APPLICATIONS INFORMATION
V
CC
corresponding to MOSFET’s VGS value (VCC in this case)
can be readily obtained from the manafacturer’s QGS vs
VGS curves:
LTC1693
Load Capacitive Power (MOS) = (VCC)(QG)(f)
INPUT SIGNAL
GOING BEL0W
GND PIN
IN
R1
D1
Transition state power losses are due to both AC currents
required to charge and discharge the drivers’ internal
nodal capacitances and cross-conduction currents in the
internal gates.
POTENTIAL
PARASITIC
SUBSTRATE
DIODE
1693 F04
UVLO and Thermal Shutdown
GND
The LTC1693’s UVLO detector disables the input buffer
and pulls the output pin to ground if VCC < 4V. The output
remains off from VCC = 1V to VCC = 4V. This ensures that
during start-up or improper supply voltage values, the
LTC1693 will keep the output power MOSFET off.
Figure 4
Bypassing and Grounding
LTC1693requiresproperVCCbypassingandgroundingdue
to its high speed switching (ns) and large AC currents (A).
CarelesscomponentplacementandPCBtraceroutingmay
cause excessive ringing and under/overshoot.
The LTC1693 also has a thermal detector that similarly
disables the input buffer and grounds the output pin if
junction temperature exceeds 145°C. The thermal shut-
down circuit has 20°C of hysteresis. This thermal limit
helps to shut down the system should a fault condition
occur.
To obtain the optimum performance from the LTC1693:
A. Mountthebypasscapacitorsascloseaspossibletothe
VCC and GND pins. The leads should be shortened as
much as possible to reduce lead inductance. It is
recommended to have a 0.1µF ceramic in parallel with
a low ESR 4.7µF bypass capacitor.
Input Voltage Range
LTC1693’s input pin is a high impedance node and essen-
tially draws neligible input current. This simplifies the
input drive circuitry required for the input.
Forhighvoltageswitchinginaninductiveenvironment,
ensure that the bypass capacitors’ VMAX ratings are
high enough to prevent breakdown. This is especially
important for floating driver applications.
The LTC1693 typically has 1.2V of hysteresis between its
lowandhighinputthresholds. Thisincreasesthedriver’s
robustnessagainstanygroundbouncenoises. However,
care should still be taken to keep this pin from any noise
pickup, especially in high frequency switching
applications.
B. Use a low inductance, low impedance ground plane to
reduce any ground drop and stray capacitance. Re-
member that the LTC1693 switches 1.5A peak currents
and any significant ground drop will degrade signal
integrity.
In applications where the input signal swings below the
GND pin potential, the input pin voltage must be clamped
to prevent the LTC1693’s parastic substrate diode from
turning on. This can be accomplished by connecting a
seriescurrentlimitingresistorR1andashuntingSchottky
diode D1 to the input pin (Figure 4). R1 ranges from 100Ω
to 470Ω while D1 can be a BAT54 or 1N5818/9.
C. Planthegroundroutingcarefully.Knowwherethelarge
load switching current is coming from and going to.
Maintain separate ground return paths for the input pin
and output pin. Terminate these two ground traces only
at the GND pin of the driver (STAR network).
D. Keepthecoppertracebetweenthedriveroutputpinand
the load short and wide.
10
Linear [ Linear ]
