L6997S

10/30

so considering zero the output load the output voltage is given by:

(9))

(10)

(11)

4.5 Current limit

pares this value with the ILIM pin voltage value. While the current is above the current limit value, the control

inhibits the high side MOSFET Turn On.

To properly set the current limit threshold, it should be noted that this is a valley current limit. The Average cur-

The average output current in current limit is given by:

(12)

Thus, to set the current threshold, choose RILIM according to the following equation:

(13)

In overcurrent conditions the system keeps the current constant until the output voltage meets the undervoltage

threshold. The negative valley current limit, for the sink mode, is set automatically at the same value of the pos-

itive valley current limit. The average negative current limit differs from the positive average current limit by the

ripple current; this difference is due to the valley control technique.

The current limit system accuracy is function of the precision of the resistance connected to the ILIM pin and

to ensure the system linearity.

Figure 9. Current limit schematic

V

()

Qt,C

SS

()

C

out

-------------------------

R

K

ILIM

I

SS

t

2

⋅⋅

⋅

()

C

out

V

SS

C

SS

2

⋅⋅

∆

⋅

()

-----------------------------------------------------------------------------

==

V

()

V

out

C

out

V

SS

C

SS

2

⋅⋅

∆

⋅⋅

()

R

K

ILIM

I

SS

⋅⋅

()

----------------------------------------------------------------------------

0.5

=

I

OUT

CL

I

max valley

I

∆

2

-----

+

=

I

max valley

R

ILim

Rds

on

----------------- K

ILIM

⋅

=

Positive and negative current limit

R

ILIM

5µA

Current

Comparator

To

logic

PGN

D

PHASE

To inductor

LS