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10nF

OUT

15W

5kW

OUT

a) Gate-Controlled Current Limit b) Serial Current Limit

VSP

Switch

3kW

16V

GND

XTR111

SBOS375C –NOVEMBER 2006– REVISED JUNE 2011

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the OD pin high disables the gate driver and closes a

switch connecting an internal 3kΩ resistor from the

VSP pin to the VG pin. This resistor discharges the

gate of the external FET and closes the channel; see

Figure 38.

Table 1 lists some example devices in SO-compatible

packages, but other devices can be used as well.

Avoid external capacitance from IS. This capacitance

could be compensated by adding additional

capacitance from VG to IS; however, this

compensation may slow the output down.

The drain-to-source breakdown voltage should be

selected high enough for the application. Surge

voltage protection might be required for negative

Figure 37. External Current Limit Circuits

over-voltages. For positive over-voltages, a clamp

diode to the 24V supply is recommended, protecting

the FET from reversing.

EXTERNAL MOSFET

The XTR111 delivers the precise output current to the

IS pin. The voltage at this pin is normally 1.4V below

VSP

This output requires an external transistor (Q

EXT

) that

forms a cascode for the current output. The transistor

must be rated for the maximum possible voltage on

OUT

and must dissipate the power generated by the

current and the voltage across it.

The gate drive (VG) can drive from close to the

positive supply rail to 16V below the positive supply

Figure 38. Equivalent Circuit for Gate Drive and

voltage (V

VSP

). Most modern MOSFETs accept a

Disable Switch

maximum V

of 20V. A protection clamp is only

required if a large drain gate capacitance can pulse

the gate beyond the rating of the MOSFET. Pulling

Table 1. P-Channel MOSFET (Examples)

(1)

MANUFACTURER PART NO. BREAKDOWN VGS PACKAGE C-GATE

Infineon BSP170P –60V SOT-223 328pF

NEC 2SJ326-Z –60V Spec. 320pF

ON Semiconductor NTF2955 –60V SOT-223 492pF

Supertex Inc. TP2510 –100V TO-243AA 80pF

(1) Data from published product data sheet; not ensured.