CS5166

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18

deviation from the nominal output voltage is 25 mV smaller

than it would be if there was no droop resistor. Similarly at

full load the regulator output is pre−positioned at 18 mV

below the nominal voltage before a load turn−off. The total

voltage increase due to a load turn−off is ΔV−18 mV and the

deviation from the nominal output voltage is 18 mV smaller

than it would be if there was no droop resistor. This is

because the output capacitors are pre−charged to value that

is either 25 mV above the nominal output voltage before a

load turn−on or, 18 mV below the nominal output voltage

before a load turn−off (see Figure 16).

Obviously, the larger the voltage drop across the droop

resistor (the larger the resistance), the worse the DC and load

regulation, but the better the AC transient response.

+

−

Figure 30. Circuit Used to Determine the Voltage Across the Droop Resistor that will Trip the

Internal Current Sense Comparator

L

Q1

Q2

Current Limit Comparator

CS5166

Current Limit Setpoint Calculations

The following is the design equations used to set the

current limit trip point by determining the value of the

embedded PCB trace used as a current sensing element.

The current limit setpoint has to be higher than the normal

full load current. Attention has to be paid to the current rating

of the external power components as these are the first to fail

during an overload condition. The MOSFET continuous and

pulsed drain current rating at a given case temperature has

to be accounted for when setting the current limit trip point.

manufacturers’ data sheets allow the designer to determine

(junction temperature). This, in turn, will assist the designer

to set a proper current limit, without causing device

breakdown during an overload condition.

For 300 MHz Pentium II CPU the full load is 14.2 A. The

internal current sense comparator current limit voltage

We select the value of the current sensing element

(embedded PCB trace) for the minimum current limit

setpoint:

RSENSE(MAX) +

VTH(MIN)

ICL(MIN)

å RSENSE

1.29 + 55 mV

14.2 A å

RSENSE

1.29 + 3.87 mW å RSENSE + 3.0 mW

We calculate the range of load currents that will cause the

internal current sense comparator to detect and overload

condition.

From the overcurrent detection data section on page 3.

Nominal Current Limit Setpoint

VTH(TYP) + 76 mV

ICL(NOM) +

VTH(TYP)

RSENSE(NOM)

Maximum Current Limit Setpoint

Therefore, ICL(NOM) + 76 mV

3.0 mW +

25.3

VTH(MAX) + 110 mV

Therefore,

ICL(MAX) +

110 mV

RSENSE(MIN)

+

110 mV

RSENSE

0.71 +

110 mV

3.0 mW

0.71 +

51.6 A

Therefore, the range of load currents that will cause the

internal current sense comparator to detect an overload

condition through a 3.0 mΩ embedded PCB trace is: 14.2 A

condition.

There may be applications whose layout will require the

use of two extra filter components, a 510 Ω resistor in series

limit operation and the resistor value is layout dependent.