S-8351A50MC-J3JT2G Datasheet, PDF (16/51 Pages) Seiko Instruments Inc – STEP-UP, BUILT-IN / EXTERNAL FET PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER

STEP-UP, BUILT-IN / EXTERNAL FET PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER
S-8351/8352 Series
Rev.3.0_00
„ Operation
1. Step-up DC-DC Converter
The S-8351/8352 Series is a DC-DC converter that uses a pulse frequency modulation method (PFM) and features
low current consumption. This series is an especially efficient DC-DC converter at an output current of 100 μA or
lower.
In conventional fixed-duty PFM DC-DC converters, although a low duty ratio allows a lower ripple voltage when the
current load is light, the efficiency is decreased when the output load current is large. Conversely, a high duty ratio
increases the output load current and efficiency, but increases the ripple voltage when the output load current is low.
In the A, B, and D types, the duty ratio is automatically switched 75% when the output load current is high to secure
the load drive capability and 50% when the output load current is low to control the load drive capability to decrease
pulse skipping. This suppresses a drop in the ripple frequency, enabling control of the increase in the ripple voltage.
The C type adopts a 75% fixed-duty PFM method. The ripple voltage increases more than that of the duty switching
type with the load is low, but the efficiency is better.
In the A, B, and D types, the duty ratio is not rapidly changed, but rather smoothly switched in the intermediate area
between 50% and 75%. Therefore, fluctuation of the ripple voltage caused by duty switching is minimized. Figures
15, 16 show the ripple voltage characteristics versus the output current.
S-8351A30MC
100
90
80
70
60
50
40
30
20
10
0
0 20
Ta = 25°C
V_IN = 1.5 V
V_IN = 2 V
40 60 80 100
I_OUT [mA]
S-8351A50MC
140
Ta = 25°C
120
100
80
60
40
20
V_IN = 2 V
V_IN = 3 V
0
0 20 40 60 80 100 120 140 160 180
I_OUT [mA]
Figure 15 Output Current (I_OUT) vs. Ripple Voltage (V_rp-p) Figure 16 Output Current (I_OUT) vs. Ripple Voltage (V_rp-p)
Characteristics
Characteristics
These figures show that the ripple voltage decreases as the output load current (I_OUT) changes from large to small.
The ripple voltage becomes particularly small when I_OUT is in the coil current discontinuous region of 20 mA or less.
16
Seiko Instruments Inc.