RT8278 データシートの表示（PDF） - Richtek Technology
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RT8278 Datasheet PDF : 16 Pages
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RT8278
Switching Frequency vs. RRT
3000
2750
2500
2250
2000
1750
1500
1250
1000
750
500
250
0
0
200
400
600
800
RRRRTT ((kkΩΩ))
Figure 3. Switching Frequency vs. RRT
1000
Minimum On-Time Constraint
30.0
27.5
25.0
22.5
20.0
17.5
VOUT = 3.3V
15.0
12.5
10.0
VOUT = 1.8V
7.5
5.0
2.5
0.0
1000
1400
1800
2200
2600
3000
Switching Frequency (kHz)
Figure 4. Minimum On-Time Constraint to Input Voltage
3.50
3.45
3.40
3.35
3.30
3.25
3.20
3.15
3.10
3.05
3.00
2.95
2.90
2.85
2.80
0.0
Output Voltage vs. Load Current
VIN = 12V
VIN = 5V
fSW = 2.2MHz
0.4
0.8
1.2
1.6
2.0
Load Current (A)
Figure 5. Limit of Maximum Duty
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Chip Enable Operation
The EN pin is the enable input. Pull the EN pin low (<0.4V)
to shutdown the device. During shutdown mode, the
RT8278 quiescent current drops to lower than 25μA. Drive
the EN pin high (>1.4V, < 5.5V), to turn on the device. If
the EN pin is open, it will be pulled high by the internal
circuit. For external timing control (e.g.RC), the EN pin
can also be externally pulled high by adding a 100kΩ or
greater resistor from the VIN pin (see Figure 6). In some
cases, the output voltage of the RT8278 may still be under
UVP threshold when soft-start finishes. Then the RT8278
will restart again and the output voltage of the RT8278 will
rise to the regulation voltage. This phenomenon often
happens in high frequency operation and with slow rising
input voltage. It can easily be solved by adding a voltage
divider on the EN pin. The RT8278 will be enabled when
the input voltage rises close to the nominal input voltage.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance :
ΔIL
=
⎡
⎢⎣
VOUT
fSW x L
⎤
⎥⎦
x
⎡⎢⎣1−
VOUT
VIN
⎤
⎥⎦
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Higher frequency combined with smaller ripple
current is necessary to achieve high efficiency operation.
However, it requires a large inductor to achieve this goal.
For the ripple current selection, setting the maximum value
of the ripple current ΔIL = 0.24(IMAX) is a reasonable starting
point. The largest ripple current occurs at the highest VIN.
To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation :
L
=
⎡
⎢⎣
fSW
VOUT
x ΔIL(MAX)
⎤
⎥⎦
x
⎡⎢⎣1−
VOUT ⎤
VIN(MAX) ⎥⎦
The inductor's current rating (defined by that which causes
a temperature rise from 25°C ambient to 40°C) should be
greater than the maximum load current and its saturation
current should be greater than the short circuit peak current
limit. Refer to Table 2 for the suggested inductor selection.
DS8278-02 March 2011
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