CS51031(2001) 查看數據表（PDF） - ON Semiconductor
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CS51031 Datasheet PDF : 12 Pages
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CS51031
Buck Regulator Operation
A block diagram of a typical buck regulator is shown in
Figure 4. If we assume that the output transistor is initially
off, and the system is in discontinuous operation, the
inductor current IL is zero and the output voltage is at its
nominal value. The current drawn by the load is supplied by
the output capacitor CO. When the voltage across CO drops
below the threshold established by the feedback resistors R1
and R2 and the reference voltage VREF, the power transistor
Q1 switches on and current flows through the inductor to the
output. The inductor current rises at a rate determined by
(VIN – VOUT)/L. The duty cycle (or “on” time) for the
CS51031 is limited to 80%. If output voltage remains higher
than nominal during the entire COSC change time, the Q1
does not turn on, skipping the pulse.
Q1
VIN
CIN
Control
L
D1
Feedback
R1
CO
R2
RLOAD
Figure 4. Buck Regulator Block Diagram.
APPLICATIONS INFORMATION
CS51031 DESIGN EXAMPLE
Specifications 12 V to 5.0 V, 3.0 A Buck Controller
• VIN = 12 V ±20% (i.e. 14.4 V max., 12 V nom., 9.6 V
min.)
• VOUT = 5.0 V ±2%
• IOUT = 0.3 A to 3.0 A
• Output ripple voltage < 50 mV max.
• Efficiency > 80%
• fSW = 200 kHz
1) Duty Cycle Estimates
Since the maximum duty cycle D, of the CS51031 is
limited to 80% min., it is necessary to estimate the duty cycle
for the various input conditions over the complete operating
range.
The duty cycle for a buck regulator operating in a
continuous conduction mode is given by:
D
+
VOUT
VIN *
) VF
VSAT
where:
VSAT = Rds(on) × IOUT max. and Rds(on) is the value at TJ
100°C.
If VF = 0.60 V and VSAT = 0.60 V then the above equation
becomes:
DMAX
+
5.6
9.0
+
0.62
DMIN
+
5.6
13.8
+
0.40
2) Switching Frequency and On and Off Time
Calculations
Given that fSW = 200 kHz and DMAX = 0.80
T
+
1.0
fSW
+
5.0
ms
TON(max) + T DMAX + 5.0 ms 0.62 ^ 3.0 ms
TON(min) + T DMIN + 5.0 ms 0.40 ^ 2.0 ms
TOFF(max) + TON(min) + 5.0 ms * 2.0 ms + 3.0 ms
3) Oscillator Capacitor Selection
The switching frequency is set by COSC, whose value is
given by:
ǒ Ǔ COSC in pF +
95 10)6
ǒ Ǔ FSW
1
)
FSW
3 106
*
30 103 2
FSW
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