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L6997S

STEP DOWN CONTROLLER FOR LOW VOLTAGE OPERATIONS

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L6997S

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

Vo u t ( t ,CS S )

=

-Q-----(--t--,--C----S----S----)

Cout

=

(---R----i--l-i--m----/--R-----d---s---o---n----⋅---K----I--L---I--M-----⋅---I--S----S----⋅---t--2---)-

(Cout ⋅ ∆VSS ⋅ CSS ⋅ 2)

(9))

indicating with Vout the final value, the output charging time can be estimated as:

Vout(CSS) =

(---V----o---u---t---⋅---C-----o---u---t---⋅---∆---V-----S---S-----⋅---C-----S---S-----⋅---2----) 0.5

(Rilim/Rdson ⋅ KILIM ⋅ ISS)

(10)

the minimum CSS value is given imposing this condition:

Tout =TSS

(11)

4.5 Current limit

The current limit comparator senses the inductor current through the low side MOSFET RDSON drop and com-

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-

rent depends on the inductor value, VIN VOUT and switching frequency.

The average output current in current limit is given by:

IOUTCL

=

Imax

valley

+

∆----I

2

(12)

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

Imax valley

=

--R----I--L---i--m----

Rdson

⋅

KILIM

(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

the low side MOSFET RDSON accuracy. Moreover the voltage on ILIM pin must range between 10mV and 1V

to ensure the system linearity.

Figure 9. Current limit schematic

To inductor

LS

PGN

PHASE

R

D

ILIM

Current

Comparator

5µA

Positive and negative current limit

To

logic

10/30

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