MP1529 Datasheet, PDF (8/11 Pages) Monolithic Power Systems – Fixed Frequency Step-Up Converter LCD Backlight Plus Flash

TM
MP1529 – FIXED FREQUENCY STEP-UP CONVERTER LCD BACKLIGHT PLUS FLASH
Mode
Table 1—Enable Operation Table ^{(5, 6, 7)}
EN1
EN2 Strings 1 & 2: LCD Backlight LEDs String 3: Flash LEDs
Off
Backlight
Backlight plus
Preview
Flash
L
H (PWM)
H (PWM)
L
L
Off
L
On (PWM)
H
On (PWM)
H
Off
Off
Off
Preview current
Flash current
Notes:
5) Backlight and Backlight plus Preview may use PWM for backlight brightness control. Only strings 1 & 2 will respond to high speed PWM.
String 3 will remain continuously on or off depending on the mode.
6) Any combination of backlight string may be used. 1 or 2 or 1 & 2 will regulate properly. The part uses over voltage detection to determine
which strings are available to light. The remaining strings are then used for regulation. The part will not remain in an open loop mode if
there is at least one string of LEDs to light.
7) Changing from backlight plus preview to flash will have a delay equal to the soft-start timer due to the PWM filtering necessary for preview.
Switching the part to off mode then to flash will cancel the timer. This will take a few microseconds instead of the 10 milliseconds typically
associated with the timer being active.
APPLICATION INFORMATION
Setting the LED currents
Resistors RS1, RS2 and RS3 set the backlight
LED string (LED1 and LED2) current, the flash
LED string (LED3) preview current, and the
flash LED string current, respectively.
Determine each current setting resistor in kΩ by
the equations:
RS1 = ^{950 × VSET}
I_{LED _ BL}
RS2 = ^{1100 × VSET}
I_{LED _ PV}
RS3 = ^{1000 × VSET}
I_{LED _ FL}
Where V_SET is the internal reference voltage,
typically 1.216V. I_{LED_BL}, I_{LED_PV} and I_{LED_FL} are
the backlight LED string current, the flash LED
string preview current, and the flash LED string
flash current in mA, respectively.
Selecting the Input Capacitor
An input capacitor is required to supply the AC
ripple current to the inductor, while limiting
noise at the input source. This capacitor must
have low ESR, so ceramic is the best choice.
Use an input capacitor value of 4.7µF or
greater. This capacitor must be placed
physically close to the IN pin. Since it reduces
the voltage ripple seen at IN, it also reduces the
amount of EMI passed back along that line to
the other circuitry.
Selecting the Output Capacitor
A single 1µF to 4.7µF ceramic capacitor usually
provides sufficient output capacitance for most
applications. The impedance of the ceramic
capacitor at the switching frequency is
dominated by the capacitance, and so the
output voltage ripple is mostly independent of
the ESR. The output voltage ripple V_RIPPLE is
calculated as:
( ) _VRIPPLE
=
I_LED V_OUT − V_IN
V_OUT × C2 × f_SW
Where V_OUT is the output voltage, V_IN is the
input voltage, I_LED is the total LED current, C2 is
the capacitance of the output capacitor, and f_SW
is the 1.2MHz switching frequency.
MP1529 Rev. 1.0
www.MonolithicPower.com
8
11/23/2005
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2005 MPS. All Rights Reserved.