LTC1693-3CMS8 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1693-3CMS8

High Speed Single/Dual N-Channel MOSFET Drivers

Linear Technology 

LTC1693

APPLICATIONS INFORMATION

corresponding to MOSFET’s VGS value (VCC in this case)

can be readily obtained from the manafacturer’s QGS vs

VGS curves:

Load Capacitive Power (MOS) = (VCC)(QG)(f)

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.

UVLO and Thermal Shutdown

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.

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.

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.

The LTC1693 typically has 1.2V of hysteresis between its

low and high input thresholds. This increases the driver’s

robustness against any ground bounce noises. However,

care should still be taken to keep this pin from any noise

pickup, especially in high frequency switching

applications.

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

series current limiting resistor R1 and a shunting Schottky

diode D1 to the input pin (Figure 4). R1 ranges from 100Ω

to 470Ω while D1 can be a BAT54 or 1N5818/9.

VCC

LTC1693

INPUT SIGNAL

GOING BEL0W

GND PIN

POTENTIAL

R1

IN

D1

PARASITIC

SUBSTRATE

DIODE

Figure 4

GND

1693 F04

Bypassing and Grounding

LTC1693 requires proper VCC bypassing and grounding due

to its high speed switching (ns) and large AC currents (A).

Careless component placement and PCB trace routing may

cause excessive ringing and under/overshoot.

To obtain the optimum performance from the LTC1693:

A. Mount the bypass capacitors as close as possible to the

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.

For high voltage switching in an inductive environment,

ensure that the bypass capacitors’ VMAX ratings are

high enough to prevent breakdown. This is especially

important for floating driver 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.

C. Plan the ground routing carefully. Know where the large

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. Keep the copper trace between the driver output pin and

the load short and wide.

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