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參數(shù)資料
| 型號： | NCP1511FCT1G |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Up to 500 mA, High Efficiency Synchronous Step-Down DC-DC Converter |
| 中文描述： | 0.8 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PBGA9 |
| 封裝： | LEAD FREE, MICRO BUMP-9 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 162K |
| 代理商： | NCP1511FCT1G |
NCP1511
http://onsemi.com
12
APPLICATIONS INFORMATION
Component Selection
Input Capacitor Selection
In PWM operating mode, the input current is pulsating
with large switching noise. Using an input bypass capacitor
reduces the peak current transients drawn from the input
supply source, thereby reducing switching noise
significantly. The capacitance needed for the input bypass
capacitor depends on the source impedance of the input
supply. The RMS capacitor current is calculated as:
IRMS
IO D
D
(eq. 1)
where:
D = duty cycle, which equals V
out
/V
in
, and D’ = 1 D.
The maximum RMS current occurs at 50% duty cycle
with maximum output current, which is I
O,max
/2.
A low profile ceramic capacitor of 10 F should be used
for most of the cases. For effective bypass results, the input
capacitor should be placed as close as possible to the V
CC
pin.
Inductor Value Selection
Selecting the proper inductor value is based on the
desired ripple current. The relationship between the
inductance and the inductor ripple current is given by the
equation below.
Vout
Lfs
The DC current of the inductor should be at least equal
to the maximum load current plus half the ripple current to
prevent core saturation. For NCP1511, the compensation is
internally fixed and a fixed 6.8 H inductor is needed for
most of the applications. For better efficiency, choose a low
DC resistance inductor.
iL
1
Vout
Vin
(eq. 2)
Output Capacitor Selection
Selecting the proper output capacitor is based on the
desired output ripple voltage. Ceramic capacitors with low
ESR values will have the lowest output ripple voltage and
are strongly recommended. The output ripple voltage is
given by:
Vc
iL
ESR
1
4fsCout
(eq. 3)
The RMS output capacitor current is given by:
IRMS(Cout)
VO
2 3
(1
D)
L
fs
(eq. 4)
Where f
s
is the switching frequency and ESR is the
effective series resistance of the output capacitor. A low
ESR, 22 F ceramic capacitor is recommended for
NCP1511 in most of applications. For example, with TDK
C2012X5R0J226 output capacitor, the output ripple is less
than 10 mV at 300 mA.
Design Example
As a design example, assume that the NCP1511 is used
in a single lithiumion battery application. The input
voltage, V
in
, is 3.0 V to 4.2 V. Output condition is V
out
at
1.5 V with a typical load current of 120 mA and a maximum
of 300 mA. For NCP1511, the inductor has a predetermined
value, 6.8 H. The inductor ESR will factor into the overall
efficiency of the converter. The inductor needs to be
selected by the required peak current.
Equation 5 is the basic equation for an inductor and
describes the voltage across the inductor. The inductance
value determines the slope of the current of the inductor.
VL
L
Equation 5 is rearranged to solve for the change in
current for the ontime of the converter in Continuous
Conduction Mode.
(Vin
Vout)
L
diL
dt
(eq. 5)
(eq. 6)
iL,pkpk
DTs
(Vin
Vout)
L
Vin
Vout
1
fs
iL,max
IO,max
iL,pkpk
2
Utilizing Equations 6, the peaktopeak inductor current
is calculated using the following worstcase conditions.
Vin,max
4.2 V,Vout
1.5 V,fs
1 MHz20%,
L
6.8 H10%,iL,pkpk
Therefore, the inductor must have a maximum current
exceeding 405 mA.
Since the compensation is fixed internally in the IC, the
input and output capacitors as well as the inductor have a
predetermined value too: C
in
= 10 F and C
out
= 22 F. Low
ESR capacitors are needed for best performance.
Therefore, ceramic capacitors are recommended.
197 mA,iL,max
399 mA
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