RT4813 Datasheet, PDF (14/16 Pages) Richtek Technology Corporation – High Efficiency Boost Converter

RT4813
Input Capacitor Selection
At least two capacitor and capacitance is 22F with
rating voltage is 16V for DC bias 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 1F ceramic capacitor
placed as close as possible to the VIN and GND pins
of the IC is recommended.
Output Discharge Function
With the EN pin set to low, the VOUT pin is internally
connected to GND for 10ms by an internal discharge
N-MOSFET switch. After the 10ms, IC will be
true-shut down.
This feature prevents residual charge voltages on
capacitor connected to VOUT pins, which may impact
proper power up of the system.
Output Capacitor Selection
At least 22F x 2 capacitors is recommended to
improve VOUT ripple.
Output voltage ripple is inversely proportional to
COUT.
Output capacitor is selected according to output ripple
which is calculated as :
V_{RIPPLE(PP)}

t_ON

I_LOAD
C_OUT
and
t_ON

t_SW
D

t_SW

 _1

V_IN
V_OUT



therefore :
C_OUT

t_SW
 ^1

V_IN
V_OUT



I_LOAD
V_{RIPPLE(PP)}
and
t_SW

1
f_SW
The maximum VRIPPLE occurs at minimum input
voltage and maximum output load.
Valley Current Limit
RT4813 employs a valley-current limit detection
scheme to sense inductor current during the off-time.
When the loading current is increased such that the
loading is above the valley current limit threshold, the
off-time is increased until the current is decreased to
valley-current threshold. Next on-time begins after
current is decreased to valley-current threshold.
On-time is decided by (VOUT VIN) / VOUT ratio. The
output voltage decreases when further loading
current increase. The current limit function is
implemented by the scheme, refer to Figure 2.
Average Output Current Limit
RT4813 features the average output current limit to
protect the output terminal. When the load current is
over the limit, output current will be clamped.
I_IN (DC)
f
Valley Current Limit
Inductor Current
I_L
IL
=
VIN _ D
Lf
I_IN (DC)
Figure 2. Inductor Currents In Current Limit Operation
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DS4813-00 April 2016