S-8550 SII [Seiko Instruments Inc], S-8550 Datasheet - Page 13

• Manufacturer Part Number: S-8550
• Description: STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECIFICATION, PWM CONTROL SWITCHING REGULATORS
• Manufacturer: SII [Seiko Instruments Inc]

STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS
Rev.2.1
2. Capacitors (C
3. Output voltage setting resistors (R
The followings are explanation regarding the proper setting.
To use the portion how much the phase has feed back by the zero point effectively, set R
zero point goes into the higher frequency than the pole frequency of L and C
pole frequency of L and C
The transient response can be improved by setting the zero point frequency in the range of lower frequency.
However, since the gain becomes higher in the range of high frequency, the total phase of feedback loop delays
180° or more by setting the zero point frequency in the significantly lower range. As a result, the gain cannot be
0 dB or lower in the frequency range thus the operation might be unstable. Determine the proper value after the
sufficient evaluation under the actual condition.
A ceramic capacitor can be used for the input (C
impedance and averages the input current to improve efficiency. Select C
power supply to be used. The recommended capacitance is 4.7 µF for the S-8550/8551 Series when a general
lithium ion rechargeable battery is used.
Select as C
optimum capacitor selection depends on the L value, capacitance value, wiring, and application (output load).
Select C
With the S-8550/8551 Series, V
resistors across the VOUT and VSS pins. Because V
Connect divider resistors R
an effect, adjust the values of R
C
By setting the zero point (the phase feedback) by adding capacitor C
parallel, the feedback loop gains the phase margin. As a result, the stability can be obtained. In principle, to use
the portion how much the phase has feed back by the zero point effectively, define C
equation.
This equation is the reference.
The typical constants by our evaluation are in Table 8.
_01
FB
*1. The recommended parts in Table 5
connected in parallel with R
V
OUT
OUT
1.1
1.8
3.3
4.0
V
C
f
f
pole
(s) [V]
zero
OUT
after sufficient evaluation under actual use conditions.
FB
OUT
≅
≅
≅
IN
=
a capacitor with large capacitance and small ESR for smoothing the ripple voltage.
, C
2
2
2
x
x
(R
x
OUT
π
π
π
FB1
x
x
)
x
R
R
1
+ R
R
R
FB2
1
OUT
L
FB
FB1
FB1
FB
x
1
1
36
36
36
51
FB2
C
1
x
and the zero point frequency by C
[kΩ]
and R
x
OUT
C
Table 8 Constant for External Parts
)
FB1
FB1
OUT
70
FB
× 0.6
and R
is a capacitor for phase compensation.
kHz
can be set to any value by external divider resistors. Connect the divider
FB2
Seiko Instruments Inc.
as close to the IC to minimize effects from of noise. If noise does have
FB1
FB2
R
, R
so that R
FB2
43
18
FB2
8
9
[kΩ]
), capacitor for phase compensation (C
IN
) and output (C
FB1
FB
+ R
= 0.6 V typ., V
C
FB2
FB
120
100
56
68
FB
[pF]
< 100 kΩ.
and R
OUT
FB
OUT
FB1
to output voltage setting resistor R
) sides. C
L [µH]
.
can be calculated by this equation.
IN
3.3
3.3
3.3
3.3
OUT
according to the impedance of the
. The following equations are the
*1
IN
S-8550/8551 Series
FB
C
lowers the power supply
OUT
referring to the following
FB1
10
10
10
10
[µF]
and C
*1
FB
)
FB
so that the
FB1
The
13
in