S-8337AAAA-P8T1G Datasheet, PDF (20/51 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-UP, 1.2 MHz HIGH-FREQUENCY, PWM CONTROL SWITCHING REGULATOR CONTROLLER
S-8337/8338 Series
Rev.4.0_01
5. Oscillation frequency and maximum duty ratio setting resistors (ROSC, RDuty)
With the S-8337/8338 Series, the oscillation frequency can be set in a range of 286 kHz to 1.133 MHz
using external resistance. Connect a resistor across the ROSC and VSS pins. Select the resistor by
using the following equation and referring to Figure 12. However, the following equation and figure
assume that the resistance value is the desired value and show the theoretical values when the IC is
in the typical conditions. Note that fluctuations of resistance and IC are not considered.
R_OSC [kΩ] ≅
140 ^• 10³
f_OSC [kHz]
1400
1200
1000
800
600
400
200
0
0
200
400
600
R_OSC [kΩ]
Figure 12 ROSC vs. fOSC
With the S-8337 Series, the maximum duty ratio can be set in a range of 47% to 88.5% by an external
resistor. Connect the resistor across the RDuty and VSS pins. Select the resistance by using the
following equation and referring to Figure 13. The maximum duty ratio fluctuates according to the
oscillation frequency. If the value of ROSC is changed, therefore, be sure to change the value of
RDuty so that it is always in proportion to ROSC. However, the following equation and figure assume
that the resistance value is the desired value and show the theoretical values when the IC is in the
typical conditions. Note that fluctuations of resistance and IC are not considered.
R_Duty
R_OSC
(94.5 − MaxDuty)
≅
11.5
100
90
80
70
60
50
40
0
1
2
3
4
5
R_Duty/R_OSC
Figure 13 RDuty/ROSC vs. MaxDuty
Connect resistors ROSC and RDuty as close to the IC as possible.
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