AA18380
Agamem Microelectronics Inc.
PRELIMINARY
Switching Regulator Controller
■HOW TO SET THE TIME CONSTANT FOR SOFT START AND SHORT CIRCUIT DETECTION
1. Soft Start
A soft start function, which gradually increases the width of the output pulses at power on, will be applied if a
capacitor is connected to the SCP pin. This can prevent rush currents and overshoot when the power supply is
turned on.
At power on, the capacitor CPE connected to the SCP pin starts charging. The PWM comparator compares the
soft start setting voltage as a proportion of the voltage at the SCP pin with the sawtooth waveform. The
comparison controls the ON duty of the OUT pin, causing the soft start operation. On completion of soft start
operation, the voltage at the SCP pin stays low, the soft start setting voltage stays high, and the circuit enters the
output short circuit detection wait state.
Soft start time (The time until the output ON duty reaches approximately 50%)
tS [s] = 0.35 × CPE [µF]
2. Timer Latch Short Circuit Protection
When the load conditions suddenly change due to load effect, the short-circuit protection comparator outputs the
high-level signal (VOM+) and the capacitor CPE connected to the SCP terminal starts charging. When the external
capacitor CPE has been charged to approximately 0.8V, the latch circuit is set, the output terminal is fixed to low
level, and the dead-time is set to 100%. However, the latch circuit is not reset unless the power for the latch
circuit is turned off or restarted by the on/off control.
• Short circuit detection time
tPE [s] = 0.8 × CPE [µF]
■FUNCTIONAL DESCRIPTION
1. Switching Regulator Function
(1) Reference voltage circuit
This circuit is composed of a band gap circuit, and outputs a 1.25-volt temperature compensated reference
voltage. This reference voltage is stabilized when the supply voltage is 1.8 V or higher. In addition to providing
the reference voltage for the switching regulator, the circuit also sets the idle period.
(2) Sawtooth wave oscillator
This circuit generates a triangular wave like sawtooth with a peak of 0.8V (VOSCH) and a trough of 0.1V (VOSCL)
using a capacitor (for the time constant) and resistor connected to the OSC pin (pin 7). The oscillator frequency
can be set to any value by selecting appropriate values for the external capacitor and resistor, CT and RT. This
oscillator can provide a frequency in the range 10 kHz to 1 MHz.
-1
1
[HZ]
=0.48×
f
OSC =
VOSCL
VOSCH
CT × RT
CT × RT × ln
(3) Error amplifier
This error amplifier detects and amplifies the DC-DC converter output voltage, and inputs that signal to a PWM
comparator. The 0.5 V internal reference voltage is applied to the non-inverting input. Arbitrary gain and phase
compensation can be connected by inserting a resistor and capacitor in series between the error amplifier output
pin (pin 8) and the inverting input pin (pin 1).
9
©Copyright Agamem Microelectronics Inc.
www.agamem.com.tw
2007/1/15
AGAMEM [ AGAMEM MICROELECTRONIC INC. ]
