34704 datasheet(40/54 Pages) FREESCALE

40 / 54 page

Analog Integrated Circuit Device Data



Freescale Semiconductor

34704

FUNCTIONAL DEVICE OPERATION

COMPONENT CALCULATION

1. Define IOB as the minimum current to maintain CCM as

15% of full load.

2. 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

I

then:

Note: On the 34704B Use the recommended 3.0uH

inductor rated between 50 to 100 mA in order to have this

regulator working in DCM. Rising the inductor value will make

the regulator to begin working in CCM.

• 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. The minimum

capacitor value is approximately:

•Where

VO

ris the desired output voltage ripple.

• Now calculate the maximum allowed ESR to reach the

desired

VO

• 1CVG (Only Reg1): Use a 47uF capacitor from Ground to

VG.

• D1 (Only Reg1): Use a fast recovery schottky diode rated

to 10V at 1A.

Regulator 2, 4 and 5 (Synchronous Buck-Boost regulator

with external compensation)

These three regulators are 4-Switch synchronous buck-

boost voltage mode control DC-DC regulator that can

operate at various output voltage levels. Since each of the

regulators may work as a buck or a boost depending on the

operating voltages, they need to be compensated in different

ways for each situation.

Since the 34704 is meant to work using a LiIon battery, the

operating input voltage range is set from 2.7 - 4.2 V, then the

following scenarios are possible:

• NOTE: Since these 3 regulators can work as a buck or a

boost in a single application, a good practice to configure

these regulators is to compensate for a boost scenario and

then verify that the regulator is working in buck mode using

that same compensation.

Compensating for Buck operation:

• 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.

Lmin

Vo D

 1D

–



2IOB

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



(H)

where: D = Dutycycle

Vo = Output Voltage

T = Switching Period

IOB = Boundary Current to achieve CCM

Lmin

Vo T



16IOB

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



(H)

COUT

IomaxDmax

Fsw

Vo

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



(F)

where: Dmax = Maximum Dutycycle

FSW = Switching Frequency

ESR

Vo

Iomax

1Dmax

–

---------------------- I





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



[

]

Regulator

Input voltage

range

Operation

2.8 V

3.0 - 4.2

Buck

3.3 V

2.7 - 3.0

Boost

3.3 V

3.5 - 4.2

Buck

1.8 V

2.7 - 4.2

Buck

2.5 V

2.7 - 4.2

Buck

3.3 V

2.7 - 3.0

Boost

3.3 V

3.5 - 4.2

Buck

Lmin



Iomax

+RDSONLSFET RL

D

min T

2IOB

----------------------------------------------------------------------------------------------------------------------- D



MAXT

2IOB

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





[H]

where: RDSONLSFET = Body Resistance of the Lowside Fet

RL = Inductor Winding Resistance

D'Min = Minimum Off Percentage given by 1- (Vin_min/Vout_max)

D'max = Maximum Off Percentage given by 1- (Vin_max/Vout_min)

34704 Datasheet(PDF) 40 Page - Freescale Semiconductor, Inc

34704 Datasheet(HTML) 40 Page - Freescale Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Datasheet Download

Link URL