TSM108

7/13

5. Gate Drive

The Gate Drive stage is directly commanded from

the PWM output signal. The Gate Drive stage is a

totem pole Mosfet stage which bears different On

resistances in order to ensure a slower turn ON

than turn OFF of the P-Channel MOSFET. The

values of the output Gate Drive currents are given

by Isink (switch ON) and Isource (switch OFF).

The Gate Drive stage bears an integrated voltage

clamp which will prevent the P-Channel MOSFET

gate to be driven with voltages higher than 15V

(acting like a zener diode between Vcc and GD

(Gate Drive) pin.

6. Under Voltage Lock-Out, Over Voltage

Lock-Out

The UVLO and OVLO security functions aim at the

global application security.

When the Power supply decreases, there is the in-

herent risk to drive the P-Channel MOSFET with

insufficient Gate voltage, and therefore to lead the

MOSFET to linear operation, and to its destruc-

tion.

The UVLO is an input power supply voltage detec-

tion which imposes a complete switch OFF of the

P-Channel MOSFET as soon as the Power Sup-

ply decreases below UV. To avoid unwanted oscil-

lation of the MOSFET, a fixed hysteresis margin is

integrated (UVhyst).

UVLO is internally programmed to ensure 8V min

and 9V max, but thresholds can be adjusted by

adding an external voltage divider to modify the

value. The resistors typical values are given (Ru-

vh, Ruvl).

The OVLO is fixing the supply voltage at which the

device (and the external power section) is

switched OFF.

OVLO is internally programmed to ensure 32V

min. and 33V max., but it can be adjusted with an

external voltage divider.

Examples:

Let's suppose that the internally set value of the

UVLO and / or OVLO level should be modified in a

specific application, or under specific require-

ments.

6.1. UVLO decrease:

If the UVLO level needs to be lowered (UV1), an

additional resistor (Ruvh1) must be connected be-

tween UV and Vcc following the equation:

(i)

where Ruvh//Ruvh1 means that Ruvh1 is in paral-

lel to Ruvh

Solving i. we obtain:

Ruvh - Ruvl (UV1 - Vref))

As an example, if UV1 needs to be set to 6V,

Ruvh1 = 256k

Ω

6.2. UVLO increase:

If the UVLO level needs to be increased (UV2), an

additional resistor (Ruvl2) must be connected be-

tween UV and Gnd following the equation.

(ii)

where Ruvl//Ruvl2 means that Ruvl2 is in parallel

to Ruvl

Solving ii. we obtain:

Vref x (Ruvh + Ruvl))

As an example, if UV2 needs to be set to 12V,

Ruvl2 = 132k

Ω

6.3. OVLO decrease:

If the OVLO level needs to be lowered (OV1), an

additional resistor (Rovh1) must be connected be-

tween OV and Vcc following the equation:

(iii)

where Rovh//Rovh1 means that Rovh1 is in paral-

lel to Rovh

Solving iii. we obtain:

Rovh - Rovl (OV1 - Vref))

As an example, if OV1 needs to be set to 25V,

Rovh1 = 867k

Ω

6.4. OVLO increase:

If the OVLO level needs to be increased (OV2), an

additional resistor (Rovl2) must be connected be-

tween OV and Gnd following the equation.

(iv)

where Rovl//Rovl2 means that Rovl2 is in parallel

to Rovl

Solving iv. we obtain:

Vref x (Rovh + Rovl))

As an example, if OV2 needs to be set to 40V,

Rovl2 = 87k

Ω