Ap3440 – Diodes AP3440 User Manual
Page 9
AP3440
Document number: DS36691 Rev. 2 - 2
9 of 15
March 2014
© Diodes Incorporated
AP3440
A Product Line of
Diodes Incorporated
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Application Note
(Cont.)
Resistor R1 can be calculated according to equation 4.
1
803
.
0
2
1
OUT
V
R
R
……………………….. (4)
AP3440
R1
R2
V
OUT
FB
0.803V
SS
Figure 2. Voltage Divider Circuit
Synchronize Using the RT/CLK Pin
The RT/CLK pin of AP3440 is used to synchronize the converter with an external system clock referring to Figure 3. To implement the
synchronization feature in a system, connect a square wave to the RT/CLK pin with an on-time of at least 75ns. When the clock is detected on the
RT/CLK pin, a mode change occurs and the pin becomes a synchronization input. The internal amplifier is disabled. If clocking edges stop, the
internal amplifier is re-enabled and the mode returns to the frequency set by the resistor. The low level of the square wave must be lower than
0.6V and the high level higher than 1.6V typically. The synchronization frequency range is from 300kHz to 2000kHz. The rising edge of the SW is
synchronized to the falling edge of RT/CLK pin. Figure 4 shows a typical synchronizing waveform, the clock frequency is 2MHz.
AP3440
R
T
RT/CLK
Clock
Source
Figure 3. Synchronizing to a System Clock
Figure 4. Synchronizing Waveform
Constant Switching Frequency and Timing Resistor
The switching frequency of the AP3440 is adjustable over a wide range from 200kHz to 2000kHz by placing a resistor with maximum value of
1000k
Ω and minimum of 85kΩ, respectively, on the RT/CLK pin. An internal amplifier holds this pin at a fixed voltage when connecting an external
resistor to ground to set the switching frequency. The V
RT/CLK
is typically 0.5V. To determine the timing resistance for a given switching frequency,
use the equation 5.
0793
.
1
)
(
311890
)
(
kHz
f
k
R
SW
T
……………………….(5)
9393
.
0
)
(
133870
)
(
k
R
kHz
f
T
SW
………………………..(6)
To reduce the solution size one should typically set the switching frequency as high as possible, but tradeoffs of the efficiency, maximum input
voltage and minimum controllable on time should be considered.