CS5132 datasheet(16/20 Pages) ONSEMI

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where

R20 = resistance at 20°C;

α =

;

= operating temperature;

R = desired droop resistor value.

For temperature T = 50°C, the % R change = 12%.

Droop Resistor Tolerance

Tolerance due to sheet resistivity variation

±8%

Tolerance due to L/W error

Tolerance due to temperature variation

12%

Total tolerance for droop resistor

21%

In order to determine the droop resistor value the nominal

voltage drop across it at full load has to be calculated. This

voltage drop has to be such that the output voltage at full

load is above the minimum DC tolerance spec:

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 CS5132 DAC output voltage is

+2.004V < VDAC < +2.045V. In order not to exceed the DC

accuracy spec, the voltage drop developed across the resis-

tor must be calculated as follows:

VDROOP(TYP) =

= 61mV.

With the CS5132 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 output volt-

age tracking the error amplifier output voltage, including

the offset. When the full load current is delivered, a drop of

-50mV is developed across the resistor. Therefore, the regu-

lator 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 nom-

inal 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 resistor. 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.

For future “CPUs” the full load will be 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.

101mV

3.3m

Ω × 0.79

VTH(MAX)

RSENSE(NOM) × 0.79

VTH(MAX)

RSENSE(MIN)

86mV

3.3m

Ω

VTH(TYP)

RSENSE(NOM)

77mV

16A

VTH(MIN)

ICL(MIN)

+2.004V-1.93V

1.21

[VDAC(MIN)-VDC (MIN)]

1+RDROOP(TOLERANCE)

VDAC(MIN)-VDC(MIN)

1+RDROOP(TOLERANCE)

0.00393

°C

Application Information: continued

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