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33394

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

After substitution, the calculated inductance value is L = 45

µH, which gives 47 µH standard component value.

The peak–to peak ripple current value is:

∆IL = 0.345 A.

The peak inductor current is given by:

ILpk = 0.5∆IL + Io = 0.5x0.345 + 1.2 = 1.37[A]

The inductor saturation current is given by the upper value

of the 33394 internal switch current limit Ilim(max) = 3.0 A.

Considering also the inductor serial resistance, these

requirements are met, for example by the PO250.473T

inductor from Pulse Engineering, Inc.

5.2.1.2. Selecting the Catch Diode D1

The rectifier D1 current capability has to be greater than

calculated average current value.

The maximum reverse voltage stress placed upon this

rectifier D1 is given by maximum input voltage (maximum

transient battery voltage). These requirements are met, for

example by the HSM350 (3 A, 50 V) schottky diode from

Microsemi, Inc.

5.2.1.3. Selecting the Output Capacitor

The output capacitor Co should be a low ESR part,

therefore the 100

µF tantalum capacitor with 80 mΩ ESR was

chosen.

From the formula for calculating the ripple voltage:

VppESR = ∆IL x RESR = 0.345 x 0.08 = 28 [mV]

One device that meets both, the low ESR, and the

temperature stability requirements is, for example, the

TPSV107K020R0085 tantalum capacitor from AVX Corp.

Boost Converter Power Capability

The boost converter with selected components has to be

able to deliver the required power.

Due to the nature of this non–compensated PFM control

technique, the Boost converter output ripple voltage is higher

than if it utilized a typical PWM control method. Therefore the

switcher output voltage level is set higher than in the Buck

mode (in the Boost mode VPRE = +6 V), in order to maintain

a sufficient dropout voltage for the 5–volt linear regulators

(VDDH, VREFs) and to avoid unwanted Resets to the

microcontroller.

The most stringent conditions for the 33394 boost

converter occur with the lowest input voltage:

Vin(min) = 3.5 V

Io = 0.8 A

Vpre = +6 V

f = 200 kHz

Vfwd1 = Vfwd2 = 0.5 V

d = 0.75, duty cycle is fixed at 75% in boost mode

Figure 16. 33394 Switcher Topology – Boost Mode

VRES

Vin

RLOAD

–

ESR

Vfwd2

ILIM

I01

I02

DIL1 < DIL2

IL1

IL2

L1 > L2

IO1 > IO2

The input voltage drop associated with the resistance of

the internal switch Q1 and inductor series resistance can be

estimated as:

VD [ Ipk(min)

RD + 2.5 A

0.35

W + 0.875 V

Where:

VD is the voltage dissipated on the major parasitic

resistances, RDSon of the internal power switch and inductor

series resistance RL.

For the worst case conditions:

RD = RDSon(max) + RL = 0.25 + 0.1 = 0.35[Ω]

Ipk(min) is the minimum internal power switch current limit

value.

Then the equation for calculating

∆IL can be modified as

follows:

DIL +

Vin * VD

[(Vo ) Vfwd2) * (Vin * VD)]

(Vo ) Vfwd2)

+ 3.5 * 0.875

10*6

[(6

) 0.5) * (3.5 * 0.875)]

0.75

) 0.5)

0.2

106

+ 125[mA]

Then the maximum average input current can be

calculated as:

IinAve + Ipk(min) *

DIL + 2.5 * 0.125

+ 2.43[A]

Finally, the boost converter power capability has to be

higher than the required output power or:

Pin(max)

h u Pout

Where Pin(max) is the boost converter maximum input power:

h is the boost converter efficiency, in our case h is

estimated to be h = 85%, and includes switching losses of the

external power switch Q2 (MOSFET) inductor and capacitors

AC losses, and output rectifier D2 (schottky) switching

losses.

Pout is the boost converter output power, which includes

power loss of the output rectifier D2:

Pout + (Vo ) Vfwd2)

Io + (6 ) 0.5)

0.8

+ 5.2[W]

Pin + (Vin * VD)

IinAve

h +

+ (3.5 * 0.875)

2.43

0.85

+ 5.42[W]

As can be seen, the boost converter input power capability

meets the required criteria.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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