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CS5422 Datasheet(PDF) 14 Page - ON Semiconductor

Part No. CS5422
Description  Dual Out−of−Phase Synchronous Buck Controller with Current Limit
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS5422 Datasheet(HTML) 14 Page - ON Semiconductor

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14
Control IC Power Dissipation
The power dissipation of the IC varies with the MOSFETs
used, VCC, and the CS5422 operating frequency. The
average MOSFET gate charge current typically dominates
the control IC power dissipation.
The IC power dissipation is determined by the formula:
) PGATE(L)1 ) PGATE(H)2 ) PGATE(L)2
PCONTROL(IC) + ICC1VCC1 ) IBSTVBST ) PGATE(H)1
where:
PCONTROL(IC) = control IC power dissipation;
ICC1 = IC quiescent supply current;
VCC1 = IC supply voltage;
PGATE(H) = upper MOSFET gate driver (IC) losses;
PGATE(L) = lower MOSFET gate driver (IC) losses.
The upper (switching) MOSFET gate driver (IC) losses
are:
PGATE(H) + QGATE(H)
fSW
VBST
where:
PGATE(H) = upper MOSFET gate driver (IC) losses;
QGATE(H) = total upper MOSFET gate charge at VCC;
fSW = switching frequency;
The lower (synchronous) MOSFET gate driver (IC)
losses are:
PGATE(L) + QGATE(L)
fSW
VCC
where:
PGATE(L) = lower MOSFET gate driver (IC) losses;
QGATE(L) = total lower MOSFET gate charge at VCC;
fSW = switching frequency;
The junction temperature of the control IC is primarily a
function of the PCB layout, since most of the heat is removed
through the traces connected to the pins of the IC.
Current Sensing
The current supplied to the load can be sensed easily using
the IS+ and IS− pins for the output. These pins sense a
voltage, proportional to the output current, and compare it to
a fixed internal voltage threshold. When the differential
voltage exceeds 70 mV, the internal overcurrrent protection
system goes into hiccup mode. Two methods for sensing the
current are available.
Sense Resistor.
A sense resistor can be added in series
with the inductor. When the voltage drop across the sense
resistor exceeds the internal voltage threshold of 70 mV, a
fault condition is set.
The sense resistor is selected according to:
RSENSE + 0.070 V
ILIMIT
In a high current supply, the sense resistor will be a very
low value, typically less than 10 mΩ. Such a resistor can be
either a discrete component or a PCB trace. The resistance
value of a discrete component can be more precise than a
PCB trace, but the cost is also greater.
Setting the current limit using an external sense resistor is
very precise because all the values can be designed to
specific tolerances. However, the disadvantage of using a
sense resistor is its additional constant power loss and heat
generation.
Inductor ESR.
Another means of sensing current is to use
the intrinsic resistance of the inductor. A model of an
inductor reveals that the windings of an inductor have an
effective series resistance (ESR).
The voltage drop across the inductor ESR can be
measured with a simple parallel circuit: an RC integrator. If
the value of RS1 and C are chosen such that:
L
ESR +
RS1C
then the voltage measured across the capacitor C will be:
VC + ESR
ILIM
Selecting Components.
Select the capacitor C first. A
value of 0.1 μF is recommended. The value of RS1 can be
selected according to:
RS1 +
1
ESR
C
Typical values for inductor ESR range in the low m;
consult manufacturer’s datasheet for specific details.
Selection of components at these values will result in a
current limit of:
ILIM + 0.070 V
ESR
Figure 9. Inductor ESR Current Sensing
GATE(H)
VCC
Co
GATE(L)
IS+
IS−
RS1
C
ESR
L
Given an ESR value of 3.5 mΩ, the current limit becomes
20 A. If an increased current limit is required, a resistor
divider can be added.
The advantages of setting the current limit by using the
winding resistance of the inductor are that efficiency is
maximized and heat generation is minimized. The tolerance
of the inductor ESR must be factored into the design of the
current limit. Finally, one or two more components are
required for this approach than with resistor sensing.
Adding External Slope Compensation
Today’s voltage regulators are expected to meet very
stringent load transient requirements. One of the key factors
in achieving tight dynamic voltage regulation is low ESR.
Low ESR at the regulator output results in low output
voltage ripple. The consequence is, however, that very little
voltage ramp exists at the control IC feedback pin (VFB),
resulting in increased regulator sensitivity to noise and the
potential for loop instability. In applications where the


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