S-8365AAABA-M5T1Y2 Datasheet, PDF (23/63 Pages) Seiko Instruments Inc – PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-UP, SUPER-SMALL PACKAGE, 1.2 MHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.2.1_01
S-8365/8366 Series
6. Output voltage setting resistors (RFB1, RFB2), capacitor for phase compensation (CFB)
For the S-8365/8366 Series, VOUT can be set to any value by using external divider resistors. Connect the divider
resistors between the VOUT and VSS pins.
Because VFB = 0.6 V typ., VOUT can be calculated by using the following equation :
VOUT =
RFB1 + RFB2
RFB2
× 0.6
Connect divider resistors RFB1 and RFB2 as close to the IC as possible to minimize the effects of noise. If noise has
an effect, adjust the values of RFB1 and RFB2 so that RFB1 + RFB2 < 100 kΩ.
CFB, which is connected in parallel with RFB1, is a capacitor for phase compensation.
By setting the zero point (the phase feedback) by adding capacitor CFB to output voltage setting resistor RFB1 in
parallel, the phase margin increases, improving the stability of the feedback loop. To effectively use the feedback
portion of the phase based on the zero point, define CFB by using the following equation :
CFB ≅
L × COUT
3 × RFB1
×
VOUT
VDD
This equation is only a guide.
The following explains the optimum setting.
To efficiently use the feedback portion of the phase based on the zero point, specify settings so that the phase
feeds back at the zero point frequency (fzero) of RFB1 and CFB according to the phase delay at the pole frequency
(fpole) of L and COUT. The zero point frequency is generally set slightly higher than the pole frequency.
The following equations are used to determine the pole frequency of L and COUT and the zero point frequency set
using RFB1 and CFB.
fpole ≅
2×π×
1
L × COUT
×
fzero ≅
1
2 × π × RFB1 × CFB
VDD
VOUT
The transient response can be improved by setting the zero point frequency in a lower frequency range. If, however,
the zero point frequency is set in a significantly lower range, the gain increases in the range of high frequency and
the phase margin decreases. This might result in unstable operation. Determine the proper value after sufficient
evaluation with actual application.
The typical constants based on our evaluation are shown in Table 15.
VOUT(S) [V]
1.8
1.8
3.32
3.32
5.0
5.0
9.0
9.0
15.0
15.0
Table 15 Example of Constant for External Parts
VDD [V]
1.2
1.2
1.2
1.2
1.8
1.8
3.3
3.3
3.3
3.3
RFB1 [kΩ]
30
30
68
68
110
110
210
210
360
360
RFB2 [kΩ]
15
15
15
15
15
15
15
15
15
15
CFB [pF]
100
82
82
68
68
56
39
33
39
33
L [μH]
3.3
2.2
3.3
2.2
3.3
2.2
3.3
2.2
3.3
2.2
COUT [μF]
10
10
10
10
22
22
22
22
22
22
23