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MC33565D

Smart Voltage Regulator for Peripheral Card Applications

ON Semiconductor 

MC33565

OPERATING DESCRIPTION

The MC33565 is designed for power managed computer

applications such as peripheral card interface (PCI) and

network interface cards (NIC) where glitch–free transitions

between 3.3 V and 5.0 V supplies are necessary. In this type

of application, the presence of a 5.0 V supply represents the

“active” system mode, while the presence of 3.3 V

represents the “sleep” system mode. The MC33565

complies with the instantly available requirements as

specified by the Advanced Configuration and Power

Interface (ACPI) standards set by Intel, Microsoft, and

Toshiba. A regulated output voltage of 3.3 V is available

even when the 5.0 V supply has been shut down and only the

3.3 V auxiliary supply is available.

The MC33565 has two supply inputs, the Main Input

(typically 5.0 V) and an Auxiliary Input (typically 3.3 V).

The MC33565 functions as a linear regulator while the Main

Input is greater than its lower threshold voltage. Below this

threshold, the internal regulator turns off and the 3.3 V

output is supplied from the Auxiliary Input via the external

P–channel MOSFET. The P–channel MOSFET gate is

controlled by the Switch Drive Output.

Low Voltage Detector

Internal circuitry detects if the system is being powered

from the Main or the Auxiliary Input supply. During normal

operating conditions, the MC33565 is powered by the Main

Input. A regulated output voltage of 3.3 V is provided by an

internal low drop out 5.0 V to 3.3 V voltage regulator. While

in this mode, the gate of the P–channel MOSFET is driven

high, turning the MOSFET Switch OFF.

The internal Low Voltage Detector has typical upper and

lower thresholds of 4.17 V and 4.02 V, respectively. The

typical hysteresis voltage between the upper and lower

thresholds is 150 mV, providing good noise immunity.

If the Main Input supply is not available or the supply

voltage drops below the 4.02 V, the internal regulator turns

OFF and the Switch Drive goes low. This enables the

external MOSFET Switch, connecting the 3.3 V Auxiliary

supply to the load allowing the load to remain powered even

though the Main Input supply is not available. Once the

Main Input supply voltage is above 4.17 V, the MOSFET

Switch Drive goes high and the internal regulator is enabled.

The Low Voltage Detector logic is active throughout the

entire range of the Main Input supply ramp up. The Switch

Drive signal is never turned ON or OFF inappropriately

during the Main Input ramp up. The output voltage is kept

above 3.0 V while the load is biased from the Main Input

supply.

Input Blocking

The internal regulator pass device (NPN transistor)

ensures that no significant reverse current flows from Vout

to the Main Supply or Gnd while the output is powered by

the Auxiliary Supply. Reverse current is typically less than

6.0 mA over the entire operating temperature range.

P–Channel MOSFET Switch Polarity

The P–channel MOSFET drain should be connected to the

Auxiliary Input, the source to the load and the gate to the

Switch Drive Output. It is imperative that the polarity of the

P–channel MOSFET is not reversed. If it is reversed, that is

the drain connected to the load and the source connected to

the Auxiliary supply, the body diode could be forward

biased if the Auxiliary supply voltage is below Vout.

Consequently, the linear regulator would not turn OFF and

it would supply current to the Auxiliary supply rail.

External Compensation

Regulators are in nature feedback systems. As with any

feedback system, loop stability needs to be evaluated to

insure stability. The MC33565 requires an external

compensation capacitor with a minimum value of 4.7 mF for

stability. Increasing the capacitance will improve the overall

load transient response. The equivalent series resistance

(ESR) of the capacitor should be less than 0.5 W in order for

the output voltage to be maintained within tight tolerance.

Sense

The Sense Input provides tight regulation of the output

voltage while the Main supply is present even with varying

load current. To take the most benefit of the Sense input,

connect pin 6 as close as possible to the load. Use a separate

trace to connect the source of the MOSFET Switch to the

load as shown in Figure 23. This will help reduce

interference or coupling in the Sense Input generated by the

output current. The use of the Sense Input is required for

correct device operation.

Vin(A)

D

Switch Drive

8

Output

Regulator

Output

7

G

S

Sense Input

6

RL

4.7 mF

Figure 23. Voltage Regulation Using Sense Feature

Board Layout

It is recommended that the MC33565 is placed as close as

possible to the MOSFET Switch and compensation

capacitor. Use short traces to minimize extraneous signals

from being induced in the Sense Input or Switch Drive

Signals. Also, avoid routing the Sense Input close to the

input and load current paths, as well as the Ground return

path to prevent signal coupling.

The part list and board layout for a demonstration board

using the MC33565 in an SOIC–8 package are available.

The board operates with an input voltage between 4.3 V and

http://onsemi.com

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