ADP1621ARMZ-R7 datasheet(14/32 Pages) AD

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ADP1621

Rev. A | Page 14 of 32

APPLICATION INFORMATION: BOOST CONVERTER

In this section, an analysis of a boost converter is presented,

along with guidelines for component selection. A typical boost-

converter application circuit is shown in Figure 1.

DUTY CYCLE

To determine the worst-case inductor current ripple, output voltage

ripple, and slope-compensation factor, it is first necessary to

determine the system duty cycle. The duty cycle in continuous

conduction mode (CCM) is calculated by the equation

OUT

−

(1)

where VOUT is the desired output voltage, VIN is the input

voltage, and VD is the forward-voltage drop of the diode. A

typical Schottky diode has a forward-voltage drop of 0.5 V.

The GATE minimum on and off times determine the minimum

and maximum duty cycles, respectively. The minimum on and

off times are typically 180 ns and 190 ns, respectively. The

minimum and maximum duty cycles are given by

MIN

(2)

)

(

MIN

OFF

MIN

OFF

MAX

−

(3)

where DMIN is the minimum duty cycle, DMAX is the maximum duty

cycle, tON,MIN is the minimum on time, tOFF,MIN is the minimum off

time, tSW is the switching period, and fSW is the switching frequency.

Note that when the converter tries to operate at a duty cycle

lower than DMIN, pulse-skipping modulation occurs to maintain

the output voltage regulation (see the Light Load Operation

section).

SETTING THE OUTPUT VOLTAGE

The output voltage is set through a voltage divider from the

output voltage to the FB input. The feedback resistor ratio sets

the output voltage of the system. The regulation voltage at FB is

1.215 V. The output voltage is given by (see Figure 1)

⎟

⎠

⎞

⎜

⎝

⎛ +

OUT

215

(4)

The input bias current into FB is 25 nA typical, 70 nA

maximum. For a 0.1% degradation in regulation voltage and

with 70 nA bias current, R2 must be less than 18 kΩ, which

results in 68 μA of divider current. Choose the value of R1 to set

the output voltage. Using higher values for R2 results in reduced

output voltage accuracy due to the input bias current at the FB

pin, whereas lower values cause increased quiescent current

consumption.

INDUCTOR CURRENT RIPPLE

Choose a peak-to-peak inductor ripple current between 20%

and 40% of the average inductor current. A good starting point

for a design is to choose the peak-to-peak ripple current to be

30% of 1/(1 − D) times the maximum load current:

MAX

LOAD

−

(5)

where ΔIL is the peak-to-peak inductor ripple current, and ILOAD,MAX

is the maximum load current required by the application.

INDUCTOR SELECTION

The inductor value choice is important because it dictates

the inductor current ripple and therefore the voltage ripple

at the output.

The average inductor current, IL,AVE, is given by

LOAD

AVE

−

(6)

and the peak-to-peak inductor ripple current is inversely

proportional to the inductor value:

(7)

where fSW is the switching frequency, and L is the inductor value.

Assuming continuous conduction mode (CCM) operation, the

peak inductor current is given by

LOAD

−

(8)

Smaller inductor values are typically smaller in size and usually

less expensive, but increase the ripple current. Larger ripple current

also increases the power loss in the inductor core. Too large an

inductor value results in added expense and may impede load

transient responses because it reduces the effect of slope

compensation.

Assuming the ripple current is 30% of 1/(1 − D) times the max-

imum load current, a reasonable choice for the inductor value is

(

)

MAX

LOAD

−

(9)

From this starting point, modify the inductance to obtain the

right balance of size, cost, and output voltage ripple while

maintaining the inductor ripple current between 20% and 40%

of 1/(1 − D) times the maximum load current. Keep in mind

that the inductor saturation current must be greater than the

peak inductor current. Magnetically shielded inductors are

generally recommended, although they cost slightly more than

unshielded inductors.

Also, losses due to the inductor winding resistance reduce the

efficiency of the boost converter. This power loss is given by

LOAD

⎟

⎠

⎞

⎜

⎝

⎛

−

(10)

where PL,W is the power dissipation in the winding of the

inductor, and RW is the winding resistance.

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