S-8337AAAA-P8T1G Datasheet, PDF (18/51 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
4. External transistor
A bipolar (NPN) or enhancement (N-channel) MOS FET transistor can be used as the external
capacitor.
4. 1 Bipolar (NPN) type
The driving capability when the output current is increased by using a bipolar transistor is
determined by hFE and Rb of the bipolar transistor. Figure 11 shows a peripheral circuit.
V_IN
Pch
C_b
2200 pF
I_PK
R_b
EXT 1 kΩ
Nch
Figure 11 External Transistor Periphery
1 kΩ is recommended for Rb. Actually, calculate the necessary base current (Ib) from hFE of the
bipolar transistor as follows and select an Rb value lower than this.
I_b =
I_PK
h_FE
R_b =
V_IN – 0.7
I_b
–
0.4
I_EXTH
A small Rb increases the output current, but the efficiency decreases. Actually, a pulsating
current flows and a voltage drop occurs due to the wiring capacitance. Determine the optimum
value by experiment.
A speed-up capacitor (Cb) connected in parallel with Rb resistance as shown in Figure 11
decreases the switching loss and improves the efficiency.
Select Cb by observing the following equation.
1
C_b ≤ _{2π • Rb • fOSC • 0.7}
However, in practice, the optimum Cb value also varies depending on the characteristics of the
bipolar transistor employed. Therefore, determine the optimum value of Cb by experiment.
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