RT7257 Просмотр технического описания (PDF) - Richtek Technology
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RT7257 Datasheet PDF : 14 Pages
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RT7257B
Typical Application Circuit
VIN
4.5V to 18V
CSS
0.1µF
CIN
10µF x 2
2 VIN
BOOT 1
RT7257B
SW 3
CBOOT L
0.1µF 3.6µH
REN 100k
7 EN
R1
75k
8 SS
FB 5
4, 9 (Exposed Pad)
GND
COMP 6
CC
2.2nF
RC
22k
R2
24k
VOUT
3.3V
COUT
22µF x 2
CP
Open
Table 1. Suggested Components Selection
VOUT (V) R1 (kΩ) R2 (kΩ) RC (kΩ) CC (nF) L (μH)
8
27
3
51
2.2
10
5
62
11.8
33
2.2
6.8
3.3
75
24
22
2.2
3.6
2.5
25.5
12
16
2.2
3.6
1.5
10.5
12
10
2.2
2
1.2
12
24
8.2
2.2
2
1
3
12
6.8
2.2
2
COUT (μF)
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
BOOT
Bootstrap for High Side Gate Driver. Connect a 0.1μF or greater ceramic
capacitor from BOOT to SW pins.
2
VIN
Input Supply Voltage, 4.5V to 18V. Must bypass with a suitable large ceramic
capacitor.
3
SW
Switch Node. Connect this pin to an external L-C filter.
4,
9 (Exposed Pad)
GND
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
5
FB
Feedback Input. It is used to regulate the output of the converter to a set value
via an external resistive voltage divider.
Compensation Node. COMP is used to compensate the regulation control
6
COMP
loop. Connect a series RC network from COMP to GND. In some cases, an
additional capacitor from COMP to GND is required.
Enable Input Pin. A logic high enables the converter; a logic low forces the
7
EN
RT7257B into shutdown mode reducing the supply current to less than 3μA.
Attach this pin to VIN with a 100kΩ pull up resistor for automatic startup.
Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor
8
SS
from SS to GND to set the soft-start period. A 0.1μF capacitor sets the
soft-start period to 13.5ms.
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
2
is a registered trademark of Richtek Technology Corporation.
DS7257B-03 September 2012
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