RT7250A/B
Application Information
Over Voltage Protection (OVP)
The RT7250A/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 2A.
The RT7250A/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.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
Under Voltage Protection (UVP)
For the RT7250A/B, it provides Hiccup Mode Under
Voltage Protection (UVP). When the FB voltage drops
below 50% of the feedback reference voltage, the UVP
function will be triggered and the RT7250A/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.
V
OUT
R1
FB
RT7250A/B
GND
R2
Figure 1. Output Voltage Setting
Inductor Selection
The output voltage is set by an external resistive divider
according to the following equation :
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.
R1
R2
V
= V 1
FB
OUT
Where VFB is the feedback reference voltage (0.8V typ.).
V
VOUT
V
OUT
IL =
1
f L
External Bootstrap Diode
IN
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 ΔIL = 0.2(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 :
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 RT7250A/B.
Note that the external boot voltage must be lower than
5.5V
VOUT
f I
VOUT
L =
1
5V
V
L(MAX)
IN(MAX)
Table 2. Suggested Inductors for Typical
Application Circuit
BOOT
RT7250A/B
10nF
Component
Supplier
TDK
Dimensions
(mm)
SW
Series
Figure 2. External Bootstrap Diode
VLF10045
SLF12565
10 x 9.7 x 4.5
TDK
12.5 x 12.5 x 6.5
TAIYO
YUDEN
NR8040
8 x 8 x 4
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7250A/B-03 June 2018
www.richtek.com
11
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
