RT4805
Operation
Startup and Shutdown State
The RT4805 combined built-in power transistors,
synchronous rectification, and low supply current, it
provides a compact solution for system using advanced
Li-Ion battery chemistries.
When VIN 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.
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.
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.
Mode
Depiction
Condition
LIN 1
LIN 2
Linear startup 1
Linear startup 2
Boost soft-start
Boost mode
VIN > VOUT
LIN
VIN > VOUT
Fault State
Soft-Start
Boost
VOUT < VOUT(MIN)
VOUT = VOUT(MIN)
VIN > VOUT(MIN)
As the Figure 1 shown, it will enter to the Fault state as
below,
Bypass
Bypass mode
The timeout of LIN2 is over the 1024s.
LIN State
It is over the 2ms when the state changed from
Soft-start state to Boost mode.
When VIN is rising, it enters the LIN State. There are
two parts for the LIN state. It provides maximum current
for 1A to charge the COUT in LIN1, and the other one is
for 2A in LIN2. By the way, the EN is pulled high and
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
As the figure shown, if the timeout is over the
specification, it will enter the Fault 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.
EN = 1,
Vin > UVLO
LIN 1
Bypass Mode
Timeout < 512μs
Timeout > 512μs
In Bypass mode, output voltage will increase with VIN
when input voltage is rising after the soft-start state.
Soft-Start
Timeout
LIN 2
Timeout > 1024μs
Bypass Mode Operation
< 1024μs
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. VOUT will be followed
the VIN when VIN is higher than the target output
voltage. As the Figure 3 shown, it is transited by bypass
MOSFET (Q3). VOUT followed the VIN.
Timeout > 2ms
Timeout < 2ms
Fault
State
Boost
mode
Figure 1. RT4805 State Chart
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is a registered trademark of Richtek Technology Corporation.
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4
DS4805-03 April 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
