AN009 Datasheet, PDF (2/6 Pages) List of Unclassifed Manufacturers – Getting Started with the KXPS5

Figure 2 AIC1638/1639 Feedback Control
Diagram
In PFM control system, a fixed switch on time
(t_ON) is in cooperating with control oscillator
frequency to maintain the output voltage at the
required voltage level. As shown in Figure 2, a
boost voltage step-up circuit configuration is
composed with L, D, Q and C. AIC1638 acts as a
central unit to control on/off switching of Q by
monitoring the feedback output voltage.
Therefore, it can achieve a stable output voltage.
In the following article, The discontinuous
conduction operation in step-up converter is
illustrated for example of operation principle
(assume that V_F and V_CE are ignored):
1. As Q is turned on, a current i₁ passes
through inductor L. Energy is stored in
inductor and increasing. During the ON
time, current i₁ start at minimum value (I_L,
MIN^{=0) increasing linearly to maximum}
i_L,MAX. As theturn-on time (t_ON) of Q
increases, inductor current increase
proportionally also.
∆I = ILMAX − ILMIN = ^{VIN × tON} … … (1)
L
2. At the moment Q is turned off, inductor
current maintains at i_L,MAX. but inductor
voltage polarity inverse instantly and
diode D is then forward bias, allowing
current in inductor release to load.
During the off time of Q, current i₂
decreasing flows from source into output
terminal through L and D.
3. Current i₂ decreases down to 0 after
t_OPEN time. Diode D is then turned off.
AN009
∆I = ILMAX − ILMIN = ^{(VOUT − VIN) × tOPEN} ..(2)
L
4. L, D and Q stay at rest-state to wait for
next Q turn on cycle.
SW
AIC1638
Only
Figure 3 Inductor Current i_L
1.25V REF
EXT
AIC1639
Only
VOUT
GND
Figure 4 AIC1638/1639 inside Configuration
Oscillator
The frequency is designed to be 100KHZ.
Typical duty cycle ratio is 75%. On time is
calculated :
TON = ^D = ^0.75 = 7.5 (µs)… (3)
fosc 100KHz
Error Comparator
After passing through a series voltage divider
resistors, output voltage feeds back is applied to
the input terminal of internal high speed Error
Comparator. The Comparator detects whether
the output voltage achieves the required level. If
output voltage is below the reference voltage, it
will enable the oscillator signal to the control
terminal of switching transistor, transferring the
input into the output terminal to establish a
stable voltage.
2