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| 型号： | R1230D2.91B-TR |
| 英文描述： | Analog IC |
| 中文描述： | 模拟IC |
| 文件页数： | 10/20页 |
| 文件大小： | 882K |
| 代理商： | R1230D2.91B-TR |
Rev. 1.15 - 10 -
tonc =T
×
V
IN
/V
OUT
Equation 2
When ton<tonc, the mode is the discontinuous mode, and when ton=tonc, the mode is the continuous mode.
I
OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS
When P-channel Tr. of Lx is ON:
(Wherein, Ripple Current P-P value is described as I
RP
, ON resistance of P-channel Tr. and N-channel Tr. of Lx are
respectively described as Ronp and Ronn, and the DC resistor of the inductor is described as R
L
.)
V
IN
=V
OUT
+(Ronp+R
L
)
×
I
OUT
+L
×
I
RP
/ton
Equation 3
When P-channel Tr. of Lx is “OFF”(N-channel Tr. is “ON”):
L
×
I
RP
/toff = R
L
×
I
OUT
+ V
OUT
+ Ronn
×
I
OUT
Equation 4
Put Equation 4 to Equation 3 and solve for ON duty of P-channel transistor, ton/(toff+ton)=D
ON
,
D
ON
=(V
OUT
-Ronn
×
I
OUT+
R
L
×
I
OUT
)/(V
IN
+ Ronn
×
I
OUT
-Ronp
×
I
OUT
)
Equation 5
Ripple Current is as follows;
I
RP
=(V
IN
-V
OUT
-Ronp
×
I
OUT
-R
L
×
I
OUT
)
×
D
ON
/fosc/L
wherein, peak current that flows through L, and Lx Tr. is as follows;
…
Equation 6
ILmax=I
OUT
+I
RP
/2
…
Equation 7
Consider ILmax, condition of input and output and select external components.
#
The above explanation is directed to the calculation in an ideal case in continuous mode.
I
How to Adjust Output Voltage and about Phase Compensation
As for Adjustable Output type, feedback pin (V
FB
) voltage is controlled to maintain 0.8V.
Output Voltage, V
OUT
is as following equation;
V
OUT
: R1+R2=VFB: R2
V
OUT
=V
FB
×
(R1+R2)/R2
Thus, with changing the value of R1 and R2, output voltage can be set in the specified range.
In the DC/DC converter, with the load current and external components such as L and C, phase might be behind 180
degree. In this case, the phase margin of the system will be less and stability will be worse. To prevent this, phase
margin should be secured with proceeding the phase. A pole is formed with external components L and C
OUT
.
Fpole
～
1/2
π√
LC
OUT
A zero (signal back to zero) is formed with R1 and Cb.
Fzero
～
1/(2
π×
R1
×
Cb)
First, choose the appropriate value of R1, R2 and Cb.
Set R1+R2 value 100k
or less.
For example, if L=10
μ
H, C
OUT
=10
μ
F, the cut off frequency of the pole is approximately 16kHz.
To make the cut off frequency of the zero as much as 16kHz, set R1=42k
and Cb=100pF.
If V
OUT
is set at 1.5V, R2=48k
is appropriate.
If a ceramic capacitor is desirable as C
OUT
in your application, nonetheless of the usage of both the fixed output
voltage type and adjustable output type, add 0.2
or more resistance to compensate the ESR.
Further, if a ceramic capacitor is desirable to use as C
OUT
without adding another resister to compensate the ESR,
phase should be back drastically. To make it, R2 value should be smaller compared to R1. As a result, the set output
voltage may be large. For example, to make V
OUT
=1.5V, constants are R1=42k
, R2=48k
, and Cb=100pF. If the
ceramic capacitor is used, under the heavy load condition, oscillation may be result. On the other hand, if R2=12k
and V
OUT
=3.6V, phase back becomes also large, and even if the device is used with a heavy load, the operation will
be stable.
Rb is effective for reducing the noise on V
FB
, however, it is not always necessary. If it is necessary, use as much as
30k
as Rb.
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