cs5422 ON Semiconductor, cs5422 Datasheet - Page 14

• Manufacturer Part Number: cs5422
• Description: Dual Outofphase Synchronous Buck Controller With Current Limit
• Manufacturer: ON Semiconductor

Control IC Power Dissipation
used, V
average MOSFET gate charge current typically dominates
the control IC power dissipation.
P CONTROL(IC) + I CC1 V CC1 ) I BST V BST ) P GATE(H)1
where:
are:
where:
losses are:
where:
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 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.
with the inductor. When the voltage drop across the sense
resistor exceeds the internal voltage threshold of 70 mV, a
fault condition is set.
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.
very precise because all the values can be designed to
The power dissipation of the IC varies with the MOSFETs
The IC power dissipation is determined by the formula:
P
I
V
P
P
The upper (switching) MOSFET gate driver (IC) losses
P
Q
f
The lower (synchronous) MOSFET gate driver (IC)
P
Q
f
The junction temperature of the control IC is primarily a
The current supplied to the load can be sensed easily using
Sense Resistor.
The sense resistor is selected according to:
In a high current supply, the sense resistor will be a very
Setting the current limit using an external sense resistor is
CC1
SW
SW
CONTROL(IC)
GATE(H)
GATE(L)
GATE(H)
GATE(L)
CC1
GATE(H)
GATE(L)
= switching frequency;
= switching frequency;
= IC quiescent supply current;
CC
= IC supply voltage;
P GATE(H) + Q GATE(H)
P GATE(L) + Q GATE(L)
, and the CS5422 operating frequency. The
= lower MOSFET gate driver (IC) losses.
= lower MOSFET gate driver (IC) losses;
= upper MOSFET gate driver (IC) losses;
= upper MOSFET gate driver (IC) losses;
= total lower MOSFET gate charge at V
= total upper MOSFET gate charge at V
) P GATE(L)1 ) P GATE(H)2 ) P GATE(L)2
= control IC power dissipation;
R SENSE + 0.070 V
A sense resistor can be added in series
I LIMIT
f SW
f SW
V CC
V BST
http://onsemi.com
CC
CC
;
;
14
specific tolerances. However, the disadvantage of using a
sense resistor is its additional constant power loss and heat
generation.
the intrinsic resistance of the inductor. A model of an
inductor reveals that the windings of an inductor have an
effective series resistance (ESR).
measured with a simple parallel circuit: an RC integrator. If
the value of R
then the voltage measured across the capacitor C will be:
value of 0.1 μF is recommended. The value of R
selected according to:
consult manufacturer’s datasheet for specific details.
current limit of:
20 A. If an increased current limit is required, a resistor
divider can be added.
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
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 (V
resulting in increased regulator sensitivity to noise and the
potential for loop instability. In applications where the
Inductor ESR.
The voltage drop across the inductor ESR can be
Selecting Components.
Typical values for inductor ESR range in the low m;
Selection of components at these values will result in a
Given an ESR value of 3.5 mΩ, the current limit becomes
The advantages of setting the current limit by using the
Today’s voltage regulators are expected to meet very
GATE(H)
V
IS+
IS−
GATE(L)
CC
Figure 9. Inductor ESR Current Sensing
S1
and C are chosen such that:
Another means of sensing current is to use
V C + ESR
R S1 +
I LIM + 0.070 V
ESR
L
RS1
L
+ R S1 C
Select the capacitor C first. A
ESR
ESR
1
ESR
I LIM
C
C
S1
can be
Co
FB
),