AA18380 Agamem Microelectronic, AA18380 Datasheet - Page 9

• Manufacturer Part Number: AA18380
• Description: Switching Regulator Controller
• Manufacturer: Agamem Microelectronic

PRELIMINARY
■
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 C
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%)
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 (V
capacitor C
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.
■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 (
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, C
oscillator can provide a frequency in the range 10 kHz to 1 MHz.
(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).
©Copyright Agamem Microelectronics Inc.
HOW TO SET THE TIME CONSTANT FOR SOFT START AND SHORT CIRCUIT DETECTION
• Short circuit detection time
t
t
S
PE
[s] = 0.35 × C
[s] = 0.8 × C
f
PE
OSC
has been charged to approximately 0.8V, the latch circuit is set, the output terminal is fixed to low
=
OM
+
PE
PE
) and the capacitor C
C
[µF]
Agamem Microelectronics Inc.
[µF]
T
× R
PE
T
connected to the SCP pin starts charging. The PWM comparator compares the
× ln
-1
V
V
OSCH
OSCL
PE
=
connected to the SCP terminal starts charging. When the external
0.48×
9
C
T
1
× R
T
[HZ]
AA18380
Switching Regulator Controller
V
www.agamem.com.tw
OSCH
) and a trough of 0.1V (
T
and R
2007/1/15
V
T
OSCL
. This
)