MC33565
http://onsemi.com
8
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 V
out
to the Main Supply or Gnd while the output is powered by
the Auxiliary Supply. Reverse current is typically less than
6.0 A 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 V
out
.
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 F 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 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.
Sense Input
V
in(A)
Figure 23. Voltage Regulation Using Sense Feature
R
L
Switch Drive
Output
6
7
8
D
S
G
Regulator
Output
4.7 F
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
