LM9061MXSLASHNOPB datasheet(19/33 Pages) TI1

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LM9061, LM9061-Q1

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SNOS738H – APRIL 1995 – REVISED JANAURY 2015

Feature Description (continued)

This voltage divider arrangement requires a mechanism to raise the ON/OFF pin above the 'On' threshold of 3.5

V minimum (3.1 V typical) when VCC is less than typically 16V. This can be accomplished with a second,

normally open, switch from the ON/OFF pin across R2 (Set), so that closing the switch will short R2 and the

voltage at the ON/OFF pin will be typically one-half of VCC. When VCC is at the minimum operating voltage of 7.0

V this will bias the ON/OFF pin to about 3.5 V causing the LM9061 to turn on. When VCC is above typically 16.5

V the resistor divider will have the ON/OFF pin biased above 3.5 V and shorting of the resistor R2 will not be

needed.

While the scaling of the external resistor values between VCC and the ON/OFF input pin, against the internal

30-k

Ω resistor, can be used to increase the startup voltage, it is important that the resistor ratio always has the

ON/OFF pin biased below the 'Off' threshold (1.5 V) when VCC falls below the minimum operating voltage of 7.0

The accuracy of this voltage divider arrangement is affected by normal manufacturing variations of the 'On' and

'Off' voltage thresholds and the value of internal resistor at the ON/OFF pin. If any application needs to detect

with greater precision when VCC is near to 7.0 V, an external voltage monitor should be used to drive the

ON/OFF pin. The external voltage monitor would also eliminate both the need for the switch to short R2 to start

the LM9061, as well as R2.

7.3.9 Increasing MOSFET Turnon Time

The ability of the LM9061 to quickly turn on the MOSFET is an important factor in the management of the

MOSFET power dissipation. Caution should be exercised when trying to increase the MOSFET turnon time by

limiting the Gate drive current. The MOSFET average dissipation, and the LM9061 Delay time, must be

recalculated with the extended switching transition time.

Figure 18 shows a method of increasing the MOSFET turnon time, without affecting the turnoff time. In this

method the Gate is charged at an exponential rate set by the added external Gate resistor and the MOSFET

Gate capacitances.

Figure 18. Increasing MOSFET Turnon Time

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LM9061MX/NOPB Datasheet(PDF) 19 Page - Texas Instruments