MD3221N 데이터 시트보기 (PDF) - Shindengen
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MD3221N Datasheet PDF : 26 Pages
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The load applied to C3 is supplemented from the
Vboot pin via D1 with each pulse cycle. This is
backed up by C4, while simultaneously stabilizing the
voltage at the Vboot pin. The capacitance of C4 should
therefore be equal to or greater than that of C3.
The voltage at the Vboot pin is controlled to 4V. As
C3 is charged with a voltage which is less than
this 4V by an amount equal to the VF of the diode D1
when the VF of this diode is high, the gate drive voltage
drops and the previous performance is then not
obtained. As the average current is in the order of a
few mA, a small signal diode is sufficient, however
it is important to avoid a diode with a high voltage
resistance and high VF. (refer to '17. Application
Circuit Exanples)
● Bootstrap Circuit
D1
C4
C3
7
24
Vboot
VB
MD3221N
11〜14
VOUT
（1）Power Saving Mode ( L cut ON )
This mode is ON when the LC and Vref pins are
connected (H). When the choke coil current is
cut-off the ON range is narrowed and the average
current drops. As the current flowing in the
MOSFET is reduced the losses at low-load are
reduced, however variation in the ON range
increases and transient response deteriorates in
proportion. As power saving is beneficial at high input
voltages, benefits at 5V are minimal and it is therefore
recommended that (2) Current Regeneration Mode
( L cut OFF ) be used at low voltages.
● L1 Current Waveform at Low-load
LC function ON
0
LC function OFF
0
● Input Current (Vi ＝20V) at Low-load
◆The L Cut (cut off detect) Function
As previously described, a continuous current
normally flows in the choke coil. This current
includes a ripple current determined by the
inductance and input / output voltage of the choke coil.
As the average value is the output current, when the
output current is less than Δ1/2 at low-load the
current becomes discontinuous (cut-off).
The MD3221N allows selection of two operating
modes in the cut-off region.
30
25
20
15
10
5
0
0
LC : off
LC : on
10
20
30
40
50
Output current (mA)
（2）Current Regeneration Mode ( L cut OFF )
This mode is OFF when the LC and GND pins are
connected (L). The current in the choke coil flows in
the reverse direction, thus regenerating energy and
resulting in a continuous current even at no-load.
14
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