RT4805A
Operation
The RT4805A combined built-in power transistors,
synchronous rectification, and low supply current, it
provides a compact solution for system using advanced
Li-Ion battery chemistries.
In boost mode, output voltage regulation is guaranteed
to a maximum load current of 2A. Quiescent current in
Shutdown mode is less than 1A, which maximizes
battery life.
Startup and Shutdown State
When V
IN
is rising and through the LIN state, it will
enter the Startup state. If EN is pulled low, any function
is turned-off in shutdown mode.
Soft-Start State
It starts to switch in Soft-start state. After the LIN state,
output voltage is rising with the internal reference
voltage.
There is a point, it will go to fault condition, if the large
output capacitor is used and the timeout is over 2ms
after the soft-start state.
Fault State
As the Figure 1 shown, it will enter to the Fault state as
below,
Mode
LIN
LIN 1
LIN 2
Depiction
Linear startup 1
Linear startup 2
Boost soft-start
Boost mode
Bypass mode
Condition
V
IN
> V
OUT
V
IN
> V
OUT
V
OUT
< V
OUT(MIN)
V
OUT
= V
OUT(MIN)
V
IN
> V
OUT(MIN)
Soft-Start
Boost
Bypass
LIN State
The timeout of LIN2 is over the 1024s.
It is over the 2ms when the state changed from
Soft-start state to Boost mode.
When V
IN
is rising, it enters the LIN State. There are
two parts for the LIN state. It provides maximum current
for 1A to charge the C
OUT
in LIN1, and the other one is
for 2A in LIN2. By the way, the EN is pulled high and
V
IN
> UVLO.
As the figure shown, if the timeout is over the
specification, it will enter the Fault mode.
EN = 1,
Vin > UVLO
It will be the high impedance between the input and
output when the fault is triggered. A restart will be start
after 1ms.
Boost Mode
There are two normal operation modes, one is the
Boost mode, and the other one is Bypass mode. In the
Boost mode, it provides the power to load by internal
synchronous switches after the soft-start state.
Bypass Mode
LIN 1
Timeout < 512μs
Timeout > 512μs
Soft-Start
Timeout
< 1024μs
Timeout > 2ms
Timeout < 2ms
Boost
mode
LIN 2
Timeout > 1024μs
Fault
State
In Bypass mode, output voltage will increase with VIN
when input voltage is rising after the soft-start state.
Bypass Mode Operation
In automatic mode, it transits from Boost mode to
Bypass mode. As the Figure 2 shown, there are three
MOSFET (Q1 to Q3). The Q1 & Q2 is for Boost mode, it
is used by Q3 for Bypass mode. V
OUT
will be followed
the V
IN
when V
IN
is higher than the target output
voltage. As the Figure 3 shown, it is transited by bypass
MOSFET (Q3). V
OUT
followed the V
IN
.
Figure 1. RT4805A State Chart
Copyright © 2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS4805A-02
April 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
