CS51313 datasheet(17/20 Pages) CHERRY

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VDROOP(TYP) =

Example: for a 450MHz Pentium®II, the DC accuracy spec

is 1.93 < VCC(CORE) < 2.07V, and the AC accuracy spec is

1.9V < VCC(CORE) < 2.1V. The CS51313 DAC output voltage

is +2.001V < VDAC < +2.049V. In order not to exceed the

DC accuracy spec, the voltage drop developed across the

resistor must be calculated as follows:

VDROOP(TYP) =

= 71mV.

With the CS51313 DAC accuracy being 1%, the internal

error amplifier’s reference voltage is trimmed so that the

output voltage will be 25mV high at no load. With no load,

there is no DC drop across the resistor, producing an out-

put voltage tracking the error amplifier output voltage,

including the offset. When the full load current is deliv-

ered, a drop of -50mV is developed across the resistor.

Therefore, the regulator output is pre-positioned at 25mV

above the nominal output voltage before a load turn-on.

The total voltage drop due to a load step is ∆V-25mV and

the deviation from the nominal output voltage is 25mV

smaller than it would be if there was no droop resistor.

Similarly at full load the regulator output is pre-positioned

at 25mV below the nominal voltage before a load turn-off.

the total voltage increase due to a load turn-off is ∆V-25mV

and the deviation from the nominal output voltage is

25mV smaller than it would be if there was no droop resis-

tor. This is because the output capacitors are pre-charged

to a value that is either 25mV above the nominal output

voltage before a load turn-on or, 25mV below the nominal

output voltage before a load turn-off .

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.

Current Limit

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 cur-

rent limit trip point.

Temperature curves on MOSFET manufacturers’ data

sheets allow the designer to determine the MOSFET drain

current at a particular VGS and TJ (junction temperature).

This, in turn, will assist the designer to set a proper current

limit, without causing device breakdown during an over-

load condition.

Let’s assume the full CPU load is 16A. The internal

current sense comparator current limit voltage limits are:

77mV < VTH < 101mV. Also, there is a 21% total variation

in RSENSE as discussed in the previous section.

We compute the value of the current sensing element

(embedded PCB trace) for the minimum current limit set-

point:

RSENSE(MIN) = RSENSE(TYP) × 0.79,

RSENSE(MAX) = RSENSE(TYP) × 1.21,

RSENSE(MAX) =

= 4.8mΩ.

We select,

RSENSE(TYP) = 3.3mΩ.

We calculate the range of load currents that will cause the

internal current sense comparator to detect an overload

condition.

Nominal Current Limit Setpoint

From the overcurrent detection data in the electrical char-

acteristics table:

VTH(TYP) = 86mV,

ICL(NOM) =

= 26A.

Maximum Current Limit Setpoint

From the overcurrent detection data in the electrical char-

acteristics table:

VTH(MAX) = 101mV,

ICL(MAX)==

= 38.7A.

Therefore, the range of load currents that will cause the

internal current sense comparator to detect an overload

condition through a 3.3mΩ embedded PCB trace is:

19.3A < ICL < 38.7A, with 26A being the nominal overload

condition.

Design Rules for Using a Droop Resistor

The basic equation for laying an embedded resistor is:

RAR = ρ ×

or R = ρ ×

where

A= W × t = cross-sectional area;

ρ= the copper resistivity (µΩ-mil);

L= length (mils);

W = width (mils);

t = thickness (mils).

(W × t)

101mV

3.3mΩ × 0.79

VTH(MAX)

RSENSE(NOM) × 0.79

VTH(MAX)

RSENSE(MIN)

86mV

3.3mΩ

VTH(TYP)

RSENSE(NOM)

77mV

16A

VTH(MIN)

ICL(MIN)

+2.001V-1.93V

1.21

[VDAC(MIN)-VDC (MIN)]

1+RDROOP(TOLERANCE)

VDAC(MIN)-VDC(MIN)

1+RDROOP(TOLERANCE)

Application Information :continued

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