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參數(shù)資料
| 型號： | NCP5424DR2G |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual Synchronous Buck Controller with Input Current Sharing |
| 中文描述： | 1.5 A DUAL SWITCHING CONTROLLER, 750 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | LEAD FREE, SOIC-16 |
| 文件頁數(shù)： | 10/18頁 |
| 文件大?。?/td> | 148K |
| 代理商： | NCP5424DR2G |
NCP5424
http://onsemi.com
10
DESIGN GUIDELINES
Definition of the design specifications
The output voltage tolerance can be affected by any or all
of the following:
1. buck regulator output voltage setpoint accuracy;
2. output voltage change due to discharging or charging
of the bulk decoupling capacitors during a load
current transient;
3. output voltage change due to the ESR and ESL of the
bulk and high frequency decoupling capacitors,
circuit traces, and vias;
4. output voltage ripple and noise.
Budgeting the tolerance is left to the designer who must
consider all of the above effects and provide an output
voltage that will meet the specified tolerance at the load.
The designer must also ensure that the regulator
component temperatures are kept within the manufacturer’s
specified ratings at full load and maximum ambient
temperature.
Selecting Feedback Divider Resistors
V
OUT
R1
R2
V
FB
Figure 7. Selecting Feedback Divider Resistors
The feedback pins (V
FB1(2)
) are connected to external
resistor dividers to set the output voltages. The error
amplifier is referenced to 1.0 V and the output voltage is
determined by selecting resistor divider values. Resistor R1
is selected based on a design tradeoff between efficiency
and output voltage accuracy. The output voltage error can be
estimated due to the bias current of the error amplifier
neglecting resistor tolerance:
10 6
1
R2 can be sized after R1 has been determined:
Error%
1
R1
100%
R2
R1V1
1
Calculating Duty Cycle
The duty cycle of a buck converter (including parasitic
losses) is given by the formula:
Duty Cycle
D
VOUT
VIN
(VHFET
VL)
VLFET
VHFET
VL
where:
V
OUT
= buck regulator output voltage;
V
HFET
= high side FET voltage drop due to R
DS(ON)
;
V
L
= output inductor voltage drop due to inductor wire
DC resistance;
V
IN
= buck regulator input voltage;
V
LFET
= low side FET voltage drop due to R
DS(ON)
.
Selecting the Switching Frequency
Selecting the switching frequency is a tradeoff between
component size and power losses. Operation at higher
switching frequencies allows the use of smaller inductor and
capacitor values. Nevertheless, it is common to select lower
frequency operation because a higher frequency results in
lower efficiency due to MOSFET gate charge losses.
Additionally, the use of smaller inductors at higher
frequencies results in higher ripple current, higher output
voltage ripple, and lower efficiency at light load currents.
The value of the oscillator resistor is designed to be
linearly related to the switching period. If the designer
prefers not to use Figure 8 to select the necessary resistor, the
following equation quite accurately predicts the proper
resistance for room temperature conditions.
21700
ROSC
fSW
2.31fSW
where:
R
OSC
= oscillator resistor in k ;
f
SW
= switching frequency in kHz.
Figure 8. Switching Frequency
10
20
30
40
50
60
100
200
300
400
500
600
700
800
R
OSC (k )
F
Selection of the Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the
inductor value will decrease output voltage ripple, but
degrade transient response. There are many factors to
consider in selecting the inductor including cost, efficiency,
EMI and ease of manufacture. The inductor must be able to
handle the peak current at the switching frequency without
saturating, and the copper resistance in the winding should
be kept as low as possible to minimize resistive power loss.
There are a variety of materials and types of magnetic
cores that could be used for this application. Among them
are ferrites, molypermalloy cores (MPP), amorphous and
powdered iron cores. Powdered iron cores are very
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