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參數(shù)資料
| 型號： | NCP5423 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | Dual Out−of−Phase Synchronous Buck Controller with Current Limit |
| 文件頁數(shù)： | 13/16頁 |
| 文件大?。?/td> | 171K |
| 代理商： | NCP5423 |
NCP5422A, NCP5423
http://onsemi.com
13
where:
T
J
= FET junction temperature;
T
A
= ambient temperature;
P
HFET(TOTAL)
= total switching (upper) FET losses;
R
JA
= upper FET junctiontoambient thermal resistance.
Selection of the Synchronous (Lower) FET
The switch conduction losses for the lower FET can be
calculated as follows:
IRMS2
RDS(ON)
[IOUT
(1
D)]2
RDS(ON)
PRMS(L)
where:
P
RMS(L)
= lower MOSFET conduction losses;
I
OUT
= load current;
D = Duty Cycle;
R
DS(ON)
= lower FET draintosource onresistance.
The synchronous MOSFET has no switching losses,
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 nonoverlap time and the
resulting power dissipation (neglecting reverse recovery
losses) can be calculated as follows:
PSWL
where:
P
SWL
= lower FET switching losses;
V
SD
= lower FET sourcetodrain voltage;
I
LOAD
= load current;
Nonoverlap time = GATE(L)toGATE(H) or
GATE(H)toGATE(L) delay (from NCP5422A data sheet
Electrical Characteristics section);
f
SW
= switching frequency.
The total power dissipation in the synchronous (lower)
MOSFET can then be calculated as:
VSD
ILOAD
nonoverlap time
fSW
PLFET(TOTAL)
PRMS(L)
PSWL
where:
P
LFET(TOTAL)
= Synchronous (lower) FET total losses;
P
RMS(L)
= Switch Conduction Losses;
P
SWL
= Switching losses.
Once the total power dissipation in the synchronous FET
is known the maximum FET switch junction temperature
can be calculated:
TJ
TA
[PLFET(TOTAL)
R JA]
where:
T
J
= MOSFET junction temperature;
T
A
= ambient temperature;
P
LFET(TOTAL)
= total synchronous (lower) FET losses;
R
JA
= lower FET junctiontoambient thermal resistance.
Control IC Power Dissipation
The power dissipation of the IC varies with the MOSFETs
used, V
CC
, and the NCP5422A 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:
P
CONTROL(IC)
= control IC power dissipation;
I
CC1
= IC quiescent supply current;
V
CC1
= IC supply voltage;
P
GATE(H)
= upper MOSFET gate driver (IC) losses;
P
GATE(L)
= lower MOSFET gate driver (IC) losses.
The upper (switching) MOSFET gate driver (IC) losses
are:
PGATE(H)
QGATE(H)
where:
P
GATE(H)
= upper MOSFET gate driver (IC) losses;
Q
GATE(H)
= total upper MOSFET gate charge at V
CC
;
f
SW
= switching frequency;
The lower (synchronous) MOSFET gate driver (IC)
losses are:
PGATE(L)
QGATE(L)
where:
P
GATE(L)
= lower MOSFET gate driver (IC) losses;
Q
GATE(L)
= total lower MOSFET gate charge at V
CC
;
f
SW
= 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.
fSW
VBST
fSW
VCC
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 overcurrent 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 m . 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.
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相關代理商/技術參數(shù) |
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