CS5155 View Datasheet(PDF) - ON Semiconductor
Part Name
Description
MFG CO.
'CS5155' PDF : 18 Pages View PDF
CS5155
The preceding equations for duty cycle can also be used
to calculate the regulator switching frequency and select the
COFF timing capacitor:
COFF + Perioid
(1 * duty cycle)
4848.5
where:
Period
+
1
switching frequency
Schottky Diode for Synchronous MOSFET
A Schottky diode may be placed in parallel with the
synchronous MOSFET to conduct the inductor current upon
turn off of the switching MOSFET to improve efficiency.
The CS5155 reference circuit does not use this device due
to it’s excellent design. Instead, the body diode of the
synchronous MOSFET is utilized to reduce cost and
conducts the inductor current. For a design operating at
200 kHz or so, the low non−overlap time combined with
Schottky forward recovery time may make the benefits of
this device not worth the additional expense (see Figure 8,
channel 2). The power dissipation in the synchronous
MOSFET due to body diode conduction can be estimated by
the following equation:
Power + VBD ILOAD conduction time switching frequency
Where VBD = the forward drop of the MOSFET body
diode. For the CS5155 demonstration board as shown in
Figure 8;
Power + 1.6 V 13 A 100 ns 233 kHz + 0.48 W
This is only 1.3% of the 36.4 W being delivered to the
load.
“Droop” Resistor for Adaptive Voltage Positioning
Adaptive voltage positioning is used to reduce output
voltage excursions during abrupt changes in load current.
Regulator output voltage is offset +40 mV when the
regulator is unloaded, and −40 mV at full load. This results
in increased margin before encountering minimum and
maximum transient voltage limits, allowing use of less
capacitance on the regulator output (see Figure 9).
To implement adaptive voltage positioning, a “droop”
resistor must be connected between the output inductor and
output capacitors and load. This is normally implemented by
a PC board trace of the following value:
RDROOP
+
80 mV
IMAX
Adaptive voltage positioning can be disabled for
improved DC regulation by connecting the VFB pin directly
to the load using a separate, non−load current carrying
circuit trace.
Input and Output Capacitors
These components must be selected and placed carefully
to yield optimal results. Capacitors should be chosen to
provide acceptable ripple on the input supply lines and
regulator output voltage. Key specifications for input
capacitors are their ripple rating, while ESR is important for
output capacitors. For best transient response, a combination
of low value/high frequency and bulk capacitors placed
close to the load will be required.
Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the
inductor value will decrease output voltage ripple, but
degrade transient response.
THERMAL MANAGEMENT
Thermal Considerations for Power
MOSFETs and Diodes
In order to maintain good reliability, the junction
temperature of the semiconductor components should be
kept to a maximum of 150°C or lower. The thermal
impedance (junction to ambient) required to meet this
requirement can be calculated as follows:
Thermal
Impedance
+
TJUNCTION(MAX) *
Power
TAMBIENT
A heatsink may be added to TO−220 components to
reduce their thermal impedance. A number of PC board
layout techniques such as thermal vias and additional copper
foil area can be used to improve the power handling
capability of surface mount components.
EMI Management
As a consequence of large currents being turned on and off
at high frequency, switching regulators generate noise as a
consequence of their normal operation. When designing for
compliance with EMI/EMC regulations, additional
components may be added to reduce noise emissions. These
components are not required for regulator operation and
experimental results may allow them to be eliminated. The
input filter inductor may not be required because bulk filter
and bypass capacitors, as well as other loads located on the
board will tend to reduce regulator di/dt effects on the circuit
board and input power supply. Placement of the power
component to minimize routing distance will also help to
reduce emissions.
2.0 μH
33 Ω
1000 pF
Figure 20. Filter Components
http://onsemi.com
13
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