AN8018SA Datasheet, PDF (13/26 Pages) Panasonic Semiconductor – 1.8-volt 2-channel step-up, step-down, or inverting DC-DC converter control IC

Voltage Regulators
AN8018SA
s Application Notes (continued)
[4] Function descriptions
1. Reference voltage block
This block is composed of the band gap circuit, and outputs the temperature compensated 1.19 V reference
voltage. The reference voltage is stabilized when the supply voltage is 1.8 V or more. The reference voltage is also
used as the reference voltage for the error amplifier 2 block.
2. Triangular wave oscillation block
The sawtooth-waveform-like triangular wave having
a peak of approximately 0.7 V and a trough of approxi-
V_OSCH ≈ 0.7 V
mately 0.2 V can be generated by connecting the timing
capacitor C_T and resistor R_T to the OSC terminal (pin 1).
The oscillation frequency can be freely set by the value of
C_T and R_T to be connected externally. The usable oscilla-
tion frequency is from 20 kHz to the maximum 1 MHz.
The triangular wave is connected with the inverting input
of PWM comparator for channel 1 side and the
noninverting input of PWM comparator for channel 2 side
t₁
Boosting
charge
t₂
Discharging
T
V_OSCL ≈ 0.2 V
within the IC inside. Rough calculation of oscillation fre-
quency can be calculated by the following equation.
Figure 1. Triangular wave oscillation waveform
1
1
f_OSC ≈ − _{CT × RT × ln VOSCL} ≈ 0.8 × _{CT × RT} [Hz]
V_OSCH
However, boosting charge time, over-shoot and under shoot quantities are not considered in the above equation.
And refer to the experimentally determined graph of the frequency characteristics provided in the main
characteristics section.
3. Error amplifier 1 block
The output voltage of DC-DC converter is detected by the PNP-transistor-input type error-amplifier and the
amplified signal is input to the PWM comparator.
Also, it is possible to perform the gain setting and the phase compensation arbitrarily by connecting a resistor
and a capacitor from the FB1 terminal (pin 5) to GND in series.
The output voltage V_OUT1 can be set by making connection as shown in figure 2.
Case of step-up output
Case of inverting output
FB2 5
V_REF V_OUT1
R1 R3
Error amplifier 1
block
IN+1 3
V_REF
R1 R3
FB2 5
Error amplifier 1
block
IN+1 3
R2 R4
IN−1 4
To PWM
comparator input ^{R2 R4}
IN−1 4
To PWM
comparator input
V_OUT1
=
R2
R1 + R2
·
R3 + R4
R4
·
V_REF
V_OUT1
V_OUT1
=
−
(V_REF
− V_IN−1)
·
R1 + R2
R1
+ V_REF
V_IN−1
=
V_REF
·
R4
R3 + R4
Figure 2. Connection method of error amplifier 1 block
4. Error amplifier 2 block
The output voltage of DC-DC converter is detected by the NPN-transistor-input type error-amplifier and the
amplified signal is input to the PWM comparator. The internal reference voltage 1.19 V is given to the noninverting
input.
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