RT4805B
Application Information
restarts if the input voltage recovers to the input
voltage UVLO high level.
Enable
The device can be enabled or disabled by the EN pin.
When the EN pin is higher than the threshold of
logic-high, the device starts operating with soft-start.
Once the EN pin is set at low, the device will be shut
down. In shutdown mode, the converter stops
switching, internal control circuitry is turned off, and
the load is disconnected from the input. This also
means that the output voltage can drop below the
input voltage during shutdown.
Thermal Shutdown
The device has a built-in temperature sensor which
monitors the internal junction temperature. If the
temperature exceeds the threshold, the device stops
operating. As soon as the IC temperature has
decreased below the threshold with a hysteresis, it
starts operating again. The built-in hysteresis is
designed to avoid unstable operation at IC
temperatures near the over-temperature threshold.
Soft-Start State
Inductor Selection
After the successful completion of the LIN state (VOUT
≥ VIN 300mV).
The recommended nominal inductance value is
1.5H.
During Soft-Start state, VOUT is ramped up by Boost
internal loop. If VOUT fails to reach target value during
the Soft-Start period for more than 2ms, a fault
condition is declared.
It is recommended to use inductor with dc saturation
current ≥ 3500mA
Input Capacitor Selection
Output Voltage Setting
At least a 10F input capacitor is recommended to
improve transient behavior of the regulator and EMI
behavior of the total power supply circuit for LX. And
at least a 1F ceramic capacitor placed as close as
possible to the VIN and GND pins of the IC is
recommended.
User can select the output voltage level by VSEL and
I2C. If the VSEL pulled low, the default is 3.15V, and if
it pulled high, the default is 3.6V.
The output voltage range is from 2.85V to 4.4V.
Power Save Mode
Output Capacitor Selection
PSM is the way to improve efficiency at light load.
At least 22F x 2 capacitors is recommended to
When the output voltage is lower than a set threshold
voltage, the converter will operate in PSM.
improve VOUT ripple.
Output voltage ripple is inversely proportional to
It raises the output voltage with several pulses until
the loop exits PSM.
COUT
.
Output capacitor is selected according to output ripple
which is calculated as :
Under-Voltage Lockout
The under-voltage lockout circuit prevents the device
from operating incorrectly at low input voltages. It
prevents the converter from turning on the power
switches under undefined conditions and prevents the
battery from deep discharge. VIN voltage must be
greater than 1.7V to enable the converter. During
operation, if VIN voltage drops below 1.6V, the
converter is disabled until the supply exceeds the
UVLO rising threshold. The RT4805B automatically
ILOAD
VRIPPLE(PP) tON
COUT
and
V
IN
tON tSW D tSW 1
VOUT
therefore :
V
ILOAD
IN
COUT tSW 1
VOUT VRIPPLE(PP)
and
1
tSW
fSW
Copyright © 2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS4805B-01 April 2017
www.richtek.com
13
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