LV52205MU Application Note

LV52205MU Application Notes
Rev-B
Page No. Update Date
1
2013/11/1
2
2013/11/1
3
2013/11/1
4
2013/11/1
5
2013/11/1
6
2013/11/1
2013/11/1
Contents
Overall composition
Explanation of the terminal
LED Currents setting and Select of control mode
Start/Shut Down sequences
Board layout
External Part Selection
Revision History
2013/11/1
< Overall composition >
LV52205 is a Boost type DC-DC convertor for White LED drive.
It integrated a MOSFET which can tolerate 42V.
The LED current is set by resistance connected to FB terminal.
We can adjust dimming for LED currents by PWM signal.
Change of LED current do not synchronize the PWM signal.
It is converted to DC current by LPF of FCAP PIN.
Fig.1 Block Diagram
1
< Explanation of the terminal >
PIN
No.
1
PIN
Sign
FB
Equivalent circuit
Explanations
Input PIN for Feedback Voltage ;
It takes the feedback so that FB
voltage becomes a certain value.
The LED current is decided with
the voltage and an external
resistance value.
2
FCAP
Filter PIN for input PWM signals ;
A capacitor to convert PWM signal
into DC is connected to this PIN.
3
4
GND
SW
GND PIN
Switching PIN ;
SW is output PIN of DC-DC
convertor.
It is used for overvoltage detection
at the time of the LED opening.
5
PWM
Input PIN for PWM control signal ;
This PIN is used for enabling and
dimming of the LED.
6
Power supply PIN (2.7V-5.5V)
VIN
2
< LED Current setting and Select of control mode >
LED current (ILED) is expressed by resistance (RFB) connected between FB (VFB) and GND
as follows.
ILED = VFB / RFB.
When we actually coordinate LED current, we control the FB voltage, because resistance RFB
is constant. Maximum FB voltage is 200mV (typ).
200
FB Voltage(mV)
150
100
50
0
0
10
20
30
40
50
60
PWM Duty(%)
Fig.2 PWM Control
3
70
80
90
100
< Start/Shut Down sequences >
1.Please input "High" into PWM PIN during the period that is longer than Ton(20uSec) to start IC.
2.IC shut down when you make PWM PIN Low longer than Toff(8.9ms) period.
PWM Control
Fig.3 Timing Diagram in PWM signal
4
< Board Layout >
The traces that carry the high-frequency switching current have to be carefully designed on the borad
in order to minimize EMI, ripple and noise in general.
The loop shown on Fig.5 corresponds to the current path when LV52204 internal switch is closed.
The thicker lines show the switching current path.
All these traces have to be short and wide enough to minimize parasitic inductance and resistance.
Fig.6 shows the current loop, when LV52205 switch is open.
Both loop areas should be as small as possible.
Capacitor C1(VBAT-GND) has to be placed as close as possible to the VBAT pin and GND pin.
The connection between SW pin to the inductor and schottky diode should be kept as short and
wide as possible.
The trace between schottky diode and the output capacitor C2 should also be as short and
wide as possible.
Capacitor C2(VOUT-GND) has to be placed as close as possible to the GND pin.
Resistor R1(FB-GND) has to be placed as close as possible to the FB pin.
Capacitor C3(FCAP-GND) has to be placed as close as possible to the FCAP pin.
5
< External Part Selection >
< Capacitor >
The ceramic capacitor from 1uF to 4.7uF is recommended as input capacitor C1.
A ceramic capacitor requires attention which capacitance value decreases to by
applying rating DC voltage.
The ceramic capacitor from 1uF to 2.2uF is recommended as output capacitor C2.
When LED become OPEN, because the OVP voltage is applied to each parts,
please use parts which can endure 50V.
< Schottky diode >
To get the optimum efficiency, LV52204 demands a low forward voltage, high-speed
and low capacitance schottky diode . Ensure that the diode average and peak current
rating exceeds the average output current and peak inductor current.
In addition, the diode’s reverse breakdown voltage must exceed the open LED protection
voltage.
< Inductor >
Three different electrical parameters need to be considered when selecting an inductor,
the value of the inductor, the saturation current and the DCR.
Calculation formula of the peak current
Ipeak_p = Iout / (n x ( 1- D )) + ( VIN x D ) / 2 x L1 x Fosc
VIN:battery voltage, IOUT:load current, L:inductor value, Fosc: OSC frequency,
D:duty cycle, n:converter efficiency varies with load current.
D = ((Vout + Vf ) - VIN ) / ( Vout + Vf )
Vout:output voltage, Vf:forward voltage of Schottky diode.
It is important to ensure that the inductor current rating is high enough such that it not saturate.
As the inductor size is reduced, the peak current for a given set of conditions increases along
with higher current ripple so it is not possible to deliver maximum output power at lower
inductor values.
DCR should be small to make efficiency better.
The inductor value from 10uH to 22uH is recommended.
6
< Revision History >
Revision
O
A
B
Date
2013/7/19
2013/9/4
2013/11/1
Discription of Revision & Writer
Initial release to refer from LV52204MU.
Addition of External Part selection as Page.6
Correction of the character font
Revision Change Originator
Tsutomu Tanaka
Nobuyuki Otaka
Tsutomu Tanaka