AT1380
Preliminary Product Information
Switching Regulator Power Controller
Setting Oscillating Frequency
The oscillator circuit generates a triangular sawtooth wave with a peak of 0.8V and a trough of 0.1V
using the timing capacitor (CT) and the timing resistor (RT) that are connected to the OSC pin. This
oscillator can provide oscillating frequency in the range from 20KHz to 1MHz. The waveform of the
OSC pin is shown as Figure 2.
Figure 2 Oscillator Waveform
The cycle (T) can be determined as follows:
t
VX
VH
VX = VHe−CT×RT
t = −CT × RT × loge (
)
T = −CT × RT × loge (0.1/ 0.8) = 2.1×CT × RT s
[ ]
Switching Regulator Function
(1) Reference voltage circuit
A temperature-compensated reference voltage (~1.25V) is generated by the reference voltage
circuit from the voltage supplied by the power supply pin (pin 3). The circuit also sets the idle
period besides providing the reference voltage for the switching regulator.
(2) Error amplifier
The error amplifier detects the output voltage of the switching regulator and sets the PWM
control signal. The voltage gain is fixed. The system is made stable by connecting a phase
compensation capacitor to the FB pin (pin 8).
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Tel: 886-3-563-0878
Fax: 886-3-563-0879
WWW: http://www.aimtron.com.tw
Email: service@aimtron.com.tw
16
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
