S8355 datasheet(30/72 Pages) SII

30 / 72 page

STEP-UP, SUPER-SMALL PACKAGE, 600 kHz, PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

S-8355/56/57/58 Series

Rev.8.1_01

4.2 Bipolar (NPN) Type

A circuit example using the CPH3210 (hFE = 200 to 560) from Sanyo Electric Co., Ltd. As a bipolar transistor

(NPN) is shown in Figure 19 to 24 in the “ Standard Circuits”. The hFE value and Rb value of the bipolar

transistor determine the driving capacity to increase the output current using a bipolar transistor. A peripheral

circuit example of the transistor is shown in Figure 18.

Nch

Pch

VOUT

IPK

EXT

2200 pF

1 k

*1.

VDD for E, G, J, K, L, M, P and Q types.

Figure 18 External Transistor Peripheral Circuit

The recommended Rb value is around 1 k

Ω. Actually, calculate the necessary base current (Ib) from the bipolar

transistor (hFE) using

, and select the smaller Rb value than

EXTH

OUT

−

*1.

A small Rb value can increase the output current, but the efficiency decreases. Since a current may flow on the

pulse and the voltage may drop due to wiring resistance or other factors in the actual circuit, therefore the

optimum Rb value should be determined by experiment.

Connecting the speed-up capacitor (Cb) in parallel with the Rb resistance as shown in Figure 18, decreases

switching loss and improves the efficiency.

The Cb value is calculated according to

osc

•

≤

Select a Cb value after performing sufficient evaluation since the optimum Cb value differs depending upon the

characteristics of the bipolar transistor.

*1.

For E, G, J, K, L, M, P and Q type,

EXTH

−

S8355 Datasheet(PDF) 30 Page - Seiko Instruments Inc