NCP5422A_06 ONSEMI [ON Semiconductor], NCP5422A_06 Datasheet - Page 13

• Manufacturer Part Number: NCP5422A_06
• Description: Dual Out−of−Phase Synchronous Buck Controller with Current Limit
• Manufacturer: ONSEMI [ON Semiconductor]

where:
Selection of the Synchronous (Lower) FET
calculated as follows:
where:
except for losses in the internal body diode, because it turns
on into near zero voltage conditions. The MOSFET body
diode will conduct during the non−overlap time and the
resulting power dissipation (neglecting reverse recovery
losses) can be calculated as follows:
where:
GATE(H)−to−GA TE(L) delay (from NCP5422A data sheet
Electrical Characteristics section);
MOSFET can then be calculated as:
where:
is known the maximum FET switch junction temperature
can be calculated:
where:
Control IC Power Dissipation
used, V
average MOSFET gate charge current typically dominates
the control IC power dissipation.
T
T
P
R
The switch conduction losses for the lower FET can be
P
I
D = Duty Cycle;
R
The synchronous MOSFET has no switching losses,
P
V
I
Non−overlap
f
The total power dissipation in the synchronous (lower)
P
P
P
Once the total power dissipation in the synchronous FET
T
T
P
R
The power dissipation of the IC varies with the MOSFETs
P SWL + V SD
OUT
LOAD
SW
A
HFET(TOTAL)
RMS(L)
SWL
LFET(TOTAL)
RMS(L)
SWL
A
LFET(TOTAL)
J
J
qJA
DS(ON)
qJA
SD
= FET junction temperature;
= MOSFET junction temperature;
P RMS(L) + I RMS 2
= ambient temperature;
= ambient temperature;
= switching frequency.
= lower FET source−to−drain voltage;
= upper FET junction−to−ambient thermal resistance.
= lower FET junction−to−ambient thermal resistance.
= load current;
CC
= lower FET switching losses;
= Switching losses.
= load current;
T J + T A ) [P LFET(TOTAL)
P LFET(TOTAL) + P RMS(L) ) P SWL
, and the NCP5422A operating frequency. The
= lower MOSFET conduction losses;
= Switch Conduction Losses;
= lower FET drain−to−source on−resistance.
+ [I OUT
= Synchronous (lower) FET total losses;
= total synchronous (lower) FET losses;
= total switching (upper) FET losses;
time
I LOAD
=
(1 * D) ] 2
R DS(ON)
GATE(L)−to−GA TE(H)
non−overlap time
R DS(ON)
R QJA ]
NCP5422A, NCP5423
f SW
http://onsemi.com
or
13
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 overcurrent 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 mW. 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
specific tolerances. However, the disadvantage of using a
sense resistor is its additional constant power loss and heat
generation.
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;
= IC supply voltage;
P GATE(H) + Q GATE(H)
P GATE(L) + Q GATE(L)
= 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
CC
CC
;
;