CS5205 datasheet(5/7 Pages) FCI

5 / 7 page

In order to meet the transient performance of the p ocessor

larger value capacitors are needed.

To limit the high

frequency noise generated by the p ocessor high quality

bypass capacitors must be used. In order to limit parasitic

inductance (ESL) and resistance (ESR) in capacitors to

acceptable limits, multiple small ceramic capacitor in

addition to high quality solid tantalum capacitors are

required.

When the adjustment terminal is bypassed to improve

the ripple rejection, the requirement for an output

capacitor increases. The Adjust pin is brought out on the

fixed voltage device specifically to allow this capability.

To further improve stability and transient response of

these devices large values of output capacitor can be

used.

The modern processors generate large high frenquency

current transients.

The CS-5205(adjustable version) develops a 1.25V

reference voltage between the Sense pin and the Adjust

pin (Figure 5).

Placing a resistor between these two

terminals causes a constant current to flow through R1

and down thouth R2 to set the output voltage. In general

R1 is chosen so that this current is the specified minimum

load current of 10 mA. The current out of the Adjust pin is

small, typically 50

A and it adds to the current from R1.

For best regulation the top of the resistor divider should

be connected directly to the Sense pin.

The load current step contains higher order frequency

components than the output coupling network must

handle until the regulator throttles to the load current

level.

Because they contain parasitic resistance and

inductance, capacitors are not ideal elements.

These

parasitic elements dominate the change in output voltage

at the beginning of a transient load step change. The

ESR of the output capacitors produces an instantaneous

step in output voltage

I(ESR). The ESL of the

output capacitors produces a droop proportional to the

rate of change of the output current V=L(

) The

output capacitance produces a change in output voltage

proportional to the time until the regulator can respond

I/C).

Figure 4 illustrates these transient

effects.

Output Voltage

If CS-5205 is connected as a single supply device with the

control and power input pins shorted together the internal

diode between the output and the power input pin will

protect the control input pin.

The CS-5205 series have internal power and thermal

limiting circuitry designed to protect the device under

overload conditions.

However maximum junction

temperature ratings should not be exceeded under

continuous normal load conditions. Careful consideration

must be given to all sources of thermal resistance from

junction to ambient, including junction-to-case, case-to-

heat sink interface and heat sink resistance itself.

Junction temperature of the Control section can run up to

125

Junction temperature of the Power section can

run up to 150

. Due to the thermal gradients between the

power transistor and the control circuitry there is a signi-

ficant difference in thermal resistance between the Control

and Power sections.

Virtually all the power dissipated by the device is dissipated

in the power transistor. The temperature rise in the power

transistor will be greater than the temperature rise in the

Control section making the thermal resistance lower in the

control section. At power levels below 12W the temperature

gradient will be less than 25

and the maximum ambient

temperature will be determined by the junction temperature

of the Control section.

Thermal Considerations

CS-5205 5A Low Dropout Positive Voltage Regulator

ESR

CAPACITANCE

SLOPE.V/t=/C

POINT AT WHICH REGU

ATOR

TAKES CONTROL

ESL

EFFECTS

Figure 4

CONTROL

VOUT

VCONTROL

VPOWER

Cs5205

ADJ

POWER

OUTPUT

SENSE

Figure 5. Setting Output Voltage

(1+R2/R1)+

ADJ

OUT

REF

IADJ

VREF

(1+R2/R1)+I

ADJ

OUT

REF

CONTROL

VOUT

VCONTROL

VPOWER

CS5205

ADJ

POWER

OUTPUT

SENSE

Figure 6. Optional Clamp Diodes Protect Against Input

CrowbarC ircuits

In normal operation CS-5205 family does not need any

protection diodes between the adjustment pin and the

output and from the output to the input to prevent die

overstress.

Internal resistors are limiting the internal

current paths on the ADJ pin. Therefore even with bypass

capacitors on the adjust pin no protection diode is

needed to ensure device safety under short-circuit

conditions. The Adjust pin can be driver on a transient

basis

7V with respect to the output without any device

degradation.

A protection diode between the Output pin and VPOWER

pin is not usually needed. Microsecond surge currents of

50A to 100A can be handled by the internal diode

between the Output pin and

In normal operations it is difficult to get those values of

surge currents even with the use of large output

capacitances.

Only with high value output capacitors,

such as 1000

to 5000

F and the

pin of the device.

POWER

pin is

POWER

instantaneously shorted to ground, damage can occour.

A diode from output to input is recommended (Figure 6).

Protection Diodes

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