RT2805A
Application Information
The RT2805A is an asynchronous high voltage buck
converter that can support the input voltage range from
5.5V to 32V and the output current can be up to 5A.
Soft-Start
The RT2805A contains an internal soft-start clamp that
gradually raises the output voltage. The typical soft-start
time is 5ms.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
Chip Enable Operation
The EN pin is the chip enable input. Pull the EN pin low
(<0.4V) will shutdown the device.During shutdown mode,
the RT2805A quiescent current drops to lower than 25μA.
Drive the EN pin to high (>1.4V, <5.5V) will turn on the
device again. If the EN pin is open, it will be pulled to high
by internal circuit. For external timing control (e.g.RC),
the ENpin can also be externally pulled to High by adding
a 100kΩ or greater resistor from the VIN pin (see Figure
3).
V
OUT
R1
FB
RT2805A
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
⎛
⎝
⎞
⎟
⎠
VOUT = VREF 1+
⎜
Where VREF is the reference voltage (1.222V typ.).
V
f ×L
VOUT
V
IN
⎡
OUT ⎤ ⎡
× 1−
⎥ ⎢
⎤
Where R1 = 100kΩ.
ΔIL =
⎢
⎣
⎥
⎦
⎦ ⎣
External Bootstrap Diode
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.
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 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 IN4148 or BAT54. The external 5V
can be a 5V fixed input from system or a 5V output of the
RT2805A.
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 :
⎡
⎤ ⎡
× 1−
⎤
V
f × ΔI
V
OUT
V
IN(MAX)
5V
OUT
L =
⎢
⎥ ⎢
⎥
L(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.
BOOT
RT2805A
SW
10nF
Figure 2. External Bootstrap Diode
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS2805A-00 November 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
