UCC27321, UCC27322

UCC37321, UCC37322

SLUS504C − SEPTEMBER 2002 − REVISED NOVEMBER 2004

8

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APPLICATION INFORMATION

The circuit in Figure 4 is utilized to test the current source capability with the output clamped to around 5 V with

UDG−01114

UCC37321

ENBL

1

2

3

4

IN

7

6

5

8

OUT

OUT

INPUT

VDD

VDD

VDD

RSNS

0.1

Ω

VSNS

100

µF

AL EL

1

µF

CER

C2

1

µF

C3

100

µF

4.5 V

DADJ

DSCHOTTKY

AGND

PGND

Figure 4. Source Current Test Circuit

It should be noted that the current sink capability is slightly stronger than the current source capability at lower

VDD. This is due to the differences in the structure of the bipolar-MOSFET power output section, where the

current source is a P-channel MOSFET and the current sink has an N-channel MOSFET.

In a large majority of applications it is advantageous that the turn-off capability of a driver is stronger than the

turn-on capability. This helps to ensure that the MOSFET is held OFF during common power supply transients

which may turn the device back ON.

operational circuit layout

It can be a significant challenge to avoid the overshoot/undershoot and ringing issues that can arise from circuit

layout. The low impedance of these drivers and their high di/dt can induce ringing between parasitic inductances

and capacitances in the circuit. Utmost care must be used in the circuit layout.

In general, position the driver physically as close to its load as possible. Place a 1-

µF bypass capacitor as close

to the output side of the driver as possible, connecting it to pins 1 and 8. Connect a single trace between the

two VDD pins (pin 1 and pin 8); connect a single trace between PGND and AGND (pin 5 and pin 4). If a ground

plane is used, it may be connected to AGND; do not extend the plane beneath the output side of the package

(pins 5 − 8). Connect the load to both OUT pins (pins 7 and 6) with a single trace on the adjacent layer to the

component layer; route the return current path for the output on the component side, directly over the output

path.

Extreme conditions may require decoupling the input power and ground connections from the output power and

ground connections. The UCCx7321/2 has a feature that allows the user to take these extreme measures, if

necessary. There is a small amount of internal impedance of about 15

Ω between the AGND and PGND pins;

there is also a small amount of impedance (

∼30 Ω) between the two VDD pins. In order to take advantage of

this feature, connect a 1-

µF bypass capacitor between VDD and PGND (pins 5 and 8) and connect a 0.1-µF

bypass capacitor between VDD and AGND (pins 1 and 4). Further decoupling can be achieved by connecting

between the two VDD pins with a jumper that passes through a 40-MHz ferrite bead and connect bias power

only to pin 8. Even more decoupling can be achieved by connecting between AGND and PGND with a pair of

anti-parallel diodes (anode connected to cathode and cathode connected to anode).