RT7257EN View Datasheet(PDF) - Richtek Technology
Part Name
Description
MFG CO.
RT7257EN Datasheet PDF : 15 Pages
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RT7257E
Preliminary
VIN
2 VIN
BOOT 1
12V
CIN
REN1 10µF x 2 RT7257E
100k
7 EN
SW 3
REN2
CBOOT
L
VOUT
8V
R1 COUT
8 SS
FB 5
CSS
4,
9 (Exposed Pad) GND
COMP 6
CC RC
CP
R2
Figure 4. The Resistors can be Selected to Set IC
Lockout Threshold
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
f ×L
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN
⎤
⎥⎦
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal.
For the ripple current selection, the value of ΔIL = 0.24(IMAX)
will be 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
=
⎡
⎢⎣
f
×
VOUT
ΔIL(MAX)
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN(MAX)
⎤
⎥⎦
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit. Please
see Table 2 for the inductor selection reference.
Table 2. Suggested Inductors for Typical
Application Circuit
C ompo nen t
Supplier
Ser ie s
D ime nsion s
(mm)
TDK
VLF10045 10 x 9.7 x 4.5
TDK
TAIYO
YUDEN
SLF12565
NR8040
12.5 x 12.5 x 6.5
8x8x4
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the high side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
IRMS
=
IOUT(MAX)
VOUT
VIN
VIN
VOUT
−1
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT / 2. This simple worst case condition is
commonly used for design because even significant
deviations do not offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design.
For the input capacitor, two 10μF low ESR ceramic
capacitors are recommended. For the recommended
capacitor, please refer to Table 3 for more details.
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response as described in a later section.
The output ripple, ΔVOUT , is determined by :
ΔVOUT
≤
ΔIL
⎡⎢⎣ESR
+
1
8fCOUT
⎤
⎥⎦
10
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