Copyright ANPEC Electronics Corp.
Rev. A.7 - Jul., 2011
APW7078
www.anpec.com.tw12
Function Description
Time
Vosc
t2t1
VH
V(t)
VL
0.9V
0.16V
t
TT
1C370
2mA
0.16V0.9V
Ct
t
V
ci
⋅=
−
=
∆
=
⋅
( )
)R72.1370(CttT
CR72.1V/VlnCRteV)t(V
TT21
TTLHTT2
T
C
T
Rt
H
+=+=
⋅== ⋅=−
Setting Oscillating Frequency
The oscillator circuit generates a triangular sawtooth wave
with a peak of 0.9V and through 0.16V using the timing
capacitor (CT) and the timing resistor (RT) that are con-
nected to OSC pin. This oscillator can provide oscillating
frequency up to 1MHz.
Setting Output Voltage
The output voltage is set using the INV pin and a re-
sistor divider connected to the output is shown in the
Typical Operating Circuit. The internal reference volt-
age is 0.5V with 2% variation, so the ratio of the feedback
resistors sets the output voltage according to the follow-
ing equation:
V5.0
2R3R
1VOUT ×
+=
To avoid the thermal noise from feedback resistor, the
resistance R2 is smaller than 100kΩ and 1% variation is
recommended.
Error Amplifier
The error amplifier detects the output voltage of the switch-
ing regulator and outputs the PWM control signal. The
voltage gain is fixed, and connecting a phase compensa-
tion resistor and capacitor to the FB pin (pin 8) provides
stable phase compensation for the system.
PWM Comparator
The voltage comparator has one inverting and three non-
inverting inputs. The comparator is a voltage/pulse width
converter that controls the ON time of the output pulse
depending on the input voltage. The output level is high
(H) when the sawtooth wave is lower than the error ampli-
fier output voltage, soft-start setting voltage, and idle pe-
riod setting voltage.
Output Circuit
The output circuit is a typical push-pull configuration to
drive an external NMOS transistor directly. It can provide a
200mA source/sink to/from OUT (pin 5).
Soft-Start and Short Circuit Detection
Soft-start operation is set by connecting capacitor CSCP to
the SCP pin (pin 2). Soft-start prevents a current spike on
start-up. On completion of the soft-start operation, the
SCP pin (pin 2) stays low and enters the short circuit
detection wait state.When an output short circuit occurs,
the error amplifier output is fixed at 1.8V and capacitor
CSCP starts charging.
After charging to approximately 0.8 V, the output pin (pin
5) is set low and the SCP pin stays low. Once the protec-
tion circuit operates, the circuit can be restored by reset-
ting the power supply. Short circuit detection time can be
calculate as below:)F(C8.0tSCP
SCP µ×=
Under-Voltage Lock Out (UVLO)
Transients during powering on or instantaneous glitches
in the supply voltage can cause system damage or
failure. The circuit prevents malfunction at low input volt-
age detects a low input voltage by comparing the supply
voltage with the internal reference voltage. On detection,
the circuit fixes the output pin to low. The system recovers
when the supply voltage rises back above the threshold
voltage of the malfunction prevention circuit.