34704 datasheet(44/54 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

34704

FUNCTIONAL DEVICE OPERATION

COMPONENT CALCULATION

The passive components associated to these frequencies are calculated with the following formulas

On the 34704 VRAMP is half of 1.2 V since each operation mode spends only half the ramp.

Regulator 3 (Synchronous Buck - internally

compensated)

• L: A buck 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:

1. Define IOB as the minimum current to maintain CCM

between 10 to 15% of full load.

• COUT: The three elements of output capacitor that

contribute to its impedance and output voltage ripple are

the ESR, the ESL and the capacitance C. A good

approach to calculate the minimum real capacitance

needed is to include the transient response analysis to

control the maximum overshoot as desired.

• First calculate the dt_I (inductor current rising time)

given by:

Where the parameter

IO

_step is the maximum current

step during the current rising time and is define as:

• Then the output capacitor can be chosen as follow:

Where

VO

max is the maximum allowed transient

overshoot expressed as a percentage of the output voltage,

typically from 3 to 5% of Vo.

• Finally find the maximum allowed ESR to allow the

desired transient response:

NOTE: do not use the parameters

VO

R and VOmax

indistinctly, the first one indicates the output voltage rip-

ple, while the second one is the maximum output volt-

age overshoot (transient response).

• R1 and RB: These two resistors help to set the output

voltage to the desire value using a VREF=0.6 V, select R1

between 10 k and 100 K and then calculate RB as follows:

Regulator 8 (Synchronous Boost - internally

compensated -Voltage or current feedback)

REG8 is a Synchronous Boost converter set to 15V with a

maximum current of 30 mA and can be used with voltage

Vinmin

VRAMP

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



min

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







 F

POR1



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





 F

Z2R1



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





 F

Z1C1



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





 F

P1C2



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





 F

P2R2



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





Lmin

Vo Iomax



RDSONLSFET RL

 D

min

T

2IOB

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



Lmin DT

2IOB

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



[H]

dtI

IomaxT

Iostep



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

[s]

Iostep



Dmax

Fsw

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



 Vinmin Vo

–

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





[A]

COUT

IomaxdtI

Vomax



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



[F]

ESRmax



r Fsw

 L



Vo 1 Dmin

–



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

[

]

Vref

------------ 1

–

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

[

]

34704 Datasheet(PDF) 44 Page - Freescale Semiconductor, Inc

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