an8049sh Panasonic Corporation of North America, an8049sh Datasheet - Page 17

• Manufacturer Part Number: an8049sh
• Description: 1.8-volt 3-channel Step-up, Step-down, And Polarity Inverting Dc-dc Converter Control Ic
• Manufacturer: Panasonic Corporation of North America

I Application Notes (continued)
[4] Function descriptions (continued)
R1
R2
5. Error amplifier 3
6. Timer latch short-circuit protection circuit
7. Low input voltage malfunction prevention circuit (U.V.L.O.)
8. PWM comparators
V
REF
V
voltage and inputs that signal to a PWM comparator. Arbitrary gain and phase compensation can be set up by
inserting a resistor and capacitor in series between the FB3 pin (pin 20) and the IN−3 pin (pin 19). The output voltage
V
against degradation or destruction if an excessive load or a short circuit of the power supply output continues for
longer than a certain fixed period.
output voltage drops and an FB pin (pins 20, 22, or 24) voltage exceeds 0.9 V, the S.C.P. comparator outputs a low
level and the timer circuit starts. This starts charging the external protection circuit delay time capacitor.
the latch circuit latches, the output drive transistors are turned off, and the dead-time is set to 100%. (See the "[5]
Time constant setup for the timer latch short-circuit protection circuit" section later in this document.)
power supply voltage falls during power on or power off.
the supply voltage level. While the supply voltage is rising, this circuit cuts off the output drive transistor until the
reference voltage reaches 1.67 V. It also sets the dead-time to 100 % and at the same time holds the S.C.P. pin (pin 1)
and the DT pins (pins 2, 3, and 4) at 0 V, and the OSC pin (pin 17) at about 1.2 V.
1 and PWM 2 comparators reverse the logic of their inputs when adjusting the on-period of their respective output.
than both of the corresponding FB pin (pins 20, 22, or 24) and the corresponding DT pin (pins 2, 3, or 4).
resistor between the DT pin and ground, or the DT pin and V
section later in this document.)
startup if a capacitor is inserted between the DT pin and ground. See the "[7] Setting the soft start time" section
later in this document.
R3
R4
OUT3
OUT3
V
This circuit is an pnp-transistor input error amplifier that detects and amplifies the DC-DC converter output
This circuit protects the external main switching elements, flywheel diodes, choke coils, and other components
The timer latch short-circuit protection circuit detects the output of the error amplifiers. If the DC-DC converter
If the error amplifier output does not return to the normal voltage range before that capacitor reaches 1.26 V,
This circuit protects the system against degradation or destruction due to incorrect control operation when the
The low input voltage malfunction prevention circuit detects the internal reference voltage that changes with
The PWM comparators control the on-period of the output pulse according to their input voltage. The PWM
The output transistors are turned on during periods when the OSC pin (pin 17) triangular waveform is lower
The maximum duty is set to 86 % internally, but can be set to a value in the range 0% to 100% by inserting a
The IC's soft start function operates to gradually increase the width of the output pulse on-period during
OUT3
can be set using the circuit shown in the figure.
=
R
1
R
+R
Step-down output
2
FB3
IN−3
IN+3
2
×
R
20
18
19
3
+R
R
4
Error
amplifier3
4
Figure 4. Connection method of error amplifier 3
× V
REF
To the PWM
comparator input
SDH00012BEB
R1
R2
V
V
OUT3
REF
REF
pin. (See the "[6] Setting the maximum duty"
V
OUT3
IN−3
FB3
IN+3
Inverting output
= −V
20
18
19
REF
Error
amplifier3
×
R
R
2
1
To the PWM
comparator input
AN8049SH
17