34704 datasheet(45/54 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34704

FUNCTIONAL DEVICE OPERATION

COMPONENT CALCULATION

feedback using the standard voltage divider configuration, or

can be programmed to work with a current feedback

configuration to control the current flowing through a LED

string. It does not need external compensation network, thus

the only components that need to be calculated are:

• L: A boost power stage can be designed to operate in

CCM for load currents above a certain level usually 5 to

15% of full load. The minimum value of inductor to

maintain CCM can be determined by using the following

procedure:

•Define IOB as the minimum current to maintain CCM as

15% of full load:

Lmin

Vo D

() 1D

–

()

2IOB

-------------------------------------------

≥

[H]

However the worst case condition for the boost power

stage is when the input voltage is equal to one half of the

output voltage, which results in the Maximum ÄIL, then:

Lmin

Vo T

()

16IOB

----------------

≥

[H]

• C

OUT: The three elements of output capacitor that

contribute to its impedance and output voltage ripple are

the ESR, the ESL and the capacitance C. The minimum

capacitor value is approximately:

COUT

IomaxDmax

Fsw Vor

-----------------------------

≥

[F]

•Where

ΔVO

r is the desired output voltage ripple.

• Now calculate

ΔVO

r the maximum allowed ESR to

reach the desired.

ESR

Vor

Iomax

1Dmax

–

-----------------------

IOB

⎝⎠

⎛⎞

---------------------------------------------

≤

[

Ω]

• R1 and RB (for Voltage feedback control): These two

resistors help to set the output voltage to the desire value

using a VREF=0.6 V, select R1 between 10k and 100K and

then calculate RB as follows:

Vref

------------

–

----------------------

[

Ω]

• RS (For current feedback control with LED string):

This resistor is attached at the end of the LED string and it

controls the amount of current flowing through it. To

calculate this resistor, set the maximum current you want

to flow though the string and use the following formula:

Vref

----------

[

Ω]

Where VREF=230 mV is the maximum internal reference

voltage in current mode control that is reflected on the FB8

pin.

When Input voltage is equal to or higher than VOUT8, a

reverse bias diode is needed from the switching node to the

output in order to cause a drop from the Input to the output,

see Figure 9 below:

CBOOT

VOUT8

VIN

BT8

SW8

VOUT8

FB8

Figure 9. Reverse Bias Diode

Regulator 7 (Inverter controller - external compensation

needed)

REG7 is a non-synchronous buck/boost inverting PWM

voltage-mode control DC-DC regulator that drive an external

P-MOSFET to supply a typical voltage of -7.0 V at a

maximum current of 60 mA.

• P-MOSFET: The peak current of the MOSFET is assumed

to be ID, which is obtained by the following formula, define

IOB from 5 to 15% of maximum current rating.

IQ ILpeak

≥

Io IOB

()

–

-----------------------------

And the voltage rating is given by:

Vin Vo

–

• Diode D7: The peak value of the diode current is IFSM

which should also be higher than ILpeak. The average

current rating should be higher than the output current low

and the repetition reverse voltage VRRM is given by:

34704 Datasheet(PDF) 45 Page - Freescale Semiconductor, Inc

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