AND8098 Datasheet, PDF (2/10 Pages) ON Semiconductor – Low-Cost 100 mA High-Voltage Buck and Buck-Boost Using NCP1052

AND8098/D
through the output and a low-frequency ripple will be found
in the output voltage. Hence, the value of C₂ is needed to be
small enough to increase this charging frequency f_VCC in
order to reduce output voltage ripple because some
efficiency is lost due to this low-frequency ripple.
I_start
FB
D₃
D
D₂ Z₂
S
D_{1 R1}
C_{1 L}
Input C₃
V_CC
D
C ^Z1
Output
C₂
(a) Buck
I_start
D₂ Z₂
D₁
FB
D₃
D
S
C₁
Input
C₃
V_CC
C₂
D
L R1 ^C
Z₁
Output
(b) Buck-boost
Figure 2. Charging Current of C₂
In Figure 2b it is noted that in the buck-boost topology the
charging current path is blocked by diode D and hence the
charging of C₂ does not affect the output voltage directly.
However, it still affects the output voltage indirectly and
slightly by adding some low-frequency noise on the
inductor. Hence, small value of C₂ is also wanted.
D_{1 R1}
C₁
V_out
(a) Buck
D₁
C₁
R₁
V_out
(b) Buck-boost
Figure 3. Output Voltage Couples to C₁ with a
Charging Current
The function of diode D₁, capacitor C₁ and resistor R₁ are
to transfer the magnitude of output voltage to a voltage
across C₁ so that the IC can regulate the output voltage. In
Figure 3, when the main switch inside the IC is opened and
the diode D is closed. In buck, the potential of the IC
reference ground (pin S) becomes almost 0 V in this
moment. In buck-boost, the potential of the IC reference
ground (pin S) becomes -V_out in this moment. The voltage
in C₁ will be charged to the output voltage. On the other
hand, when main switch is closed and the diode D is opened,
diode D₁ is reverse biased by a voltage with magnitude V_in
and V_in+V_out respectively. Hence, D₁ does not affect the
normal operation of the buck and buck-boost converter.
It is noted that the instantaneous voltage in C₁ can be
possibly greater than the output voltage especially when
output current or output ripple is too large. It directly affects
the load regulation of the circuit since the IC regulates the
output voltage based on the voltage in C₁. In order to solve
it, larger values of L and R₁ can help to slow down the
charging speed of C₁. It reduces the maximum instantaneous
voltage in C₁ so that output voltage at high output current
can be pulled up and a good regulation is made.
Larger value of L can help the load regulation but it
usually unwanted because it is bulky. Hence, resistor R₁ is
recommended. Larger value of R₁ makes higher output
voltage. Hence, it is called as a “pull-up resistor” and it can
help to pull up the output voltage slightly.
The voltage in C₁ representing the output voltage is
feedback to the feedback (FB) pin of the NCP1052 through
a diode D₂ and zener diode Z₂. When output voltage is too
high, there will be a greater-than-50 mA current inserting
into the feedback pin of the NCP1052. The NCP1052 will
stop switching when it happens. When output voltage is not
high enough, the current inserting into the feedback is
smaller than 50 mA. The NCP1052 enables switching and
power is delivered to the output until the output voltage is
too high again.
The purpose of the diode D₂ is to ensure the current is
inserting into the feedback pin because the switching of
NCP1052 can also be stopped when there is a
greater-than-50 mA current sinking from the FB pin. The
purpose of the zener diode Z₂ is to set the output voltage
threshold. The FB pin of NCP1052 with a condition of
50 mA sourcing current is about 4.3 V. The volt-drop of the
diode D₂ is loosely about 0.7 V at 50 mA. Hence, the output
voltage can be loosely set as follows:
Vout + zener ) 4.3 V ) 0.7 V
+ zener ) 5 V
(eq. 1)
According to (1), the possible minimum output voltage of
the circuit is 5.0 V when there is no zener diode Z₂.
If there is no load, the IC will automatically minimize its
duty cycle to the minimum value but the output voltage is
still possible to be very high because there is no passive
component in the circuit try to absorb the energy. As a result,
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2