rt8297a Richtek Technology Corporation, rt8297a Datasheet - Page 11

• Manufacturer Part Number: rt8297a
• Description: 1.5a, 17v, 340/800khz Synchronous Step-down Converter
• Manufacturer: Richtek Technology Corporation

Application Information
The RT8297A/B is a synchronous high voltage buck
converter that can support the input voltage range from
4V to 17V and the output current can be up to 1.5A.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
The output voltage is set by an external resistive divider
according to the following equation :
Where V
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET. It is recommended
to add an external bootstrap diode between an external
5V and the BOOT pin for efficiency improvement when
input voltage is lower than 5.5V or duty ratio is higher
than 65%. The bootstrap diode can be a low cost one
such as 1N4148 or BAT54. The external 5V can be a 5V
fixed input from system or a 5V output of the RT8297A/B.
Note that the external boot voltage must be lower than
5.5V
Over Voltage Protection (OVP)
The RT8297A/B provides Over Voltage Protection function
when output voltage over 125%. The internal MOS will be
turned off. The control will return to normal operation if
over voltage condition is removed.
DS8297A/B-00 March 2011
V
OUT
= V
FB
FB
Figure 2. External Bootstrap Diode
is the feedback reference voltage (0.8V typ.).
Figure 1. Output Voltage Setting
⎛
⎜
⎝
1
+
R2
R1
RT8297A/B
RT8297A/B
⎞
⎟
⎠
BOOT
GND
SW
FB
V
V
OUT
OUT
10nF
R1
R2
Under Voltage Protection (UVP)
Latch-Off Mode
For the RT8297A/B, it provides Hiccup Mode Under
Voltage Protection (UVP). When the FB voltage drops
below 50% of the feedback reference voltage, V
UVP function will be triggered and the RT8297A/B will
shut down for a period of time and then recover
automatically. The Hiccup Mode UVP can reduce input
current in short-circuit conditions.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current
and decreases with higher inductance.
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
highest efficiency operation. However, it requires a large
inductor to achieve this goal. For the ripple current
selection, the value of
starting point. The largest ripple current occurs at the
highest V
below the specified maximum, the inductor value should
be chosen according to the following equation :
L =
Δ
I =
L
Component
⎡
⎢
⎢
⎣
Supplier
YUDEN
f
TAIYO
Table 2. Suggested Inductors for Typical
⎡
⎢
⎣
× Δ
TDK
TDK
V
V
f L
OUT
IN
×
OUT
I
L(MAX)
. To guarantee that the ripple current stays
⎤
⎥
⎦ ⎣
⎡
⎢
1
−
⎤ ⎡
⎥ ⎢
⎥ ⎢
⎦ ⎣
Application Circuit
V
1
V
OUT
−
VLF10045
SLF12565
IN
ΔI
NR8040
V
Series
IN(MAX)
V
L
⎤
⎥
⎦
OUT
= 0.2(I
ΔI
L
increases with higher V
RT8297A/B
⎤
⎥
⎥
⎦
MAX
) will be a reasonable
12.5 x 12.5 x 6.5
10 x 9.7 x 4.5
Dimensions
8 x 8 x 4
www.richtek.com
(mm)
FB
, the
11
IN