A4402E データシートの表示（PDF） - Allegro MicroSystems

部品番号

コンポーネント説明

メーカー

A4402E

Constant On-Time Buck Converter With Integrated Linear Regulator

Allegro MicroSystems 

A4402

Constant On-Time Buck Converter

With Integrated Linear Regulator

Application Information

Switcher On-Time and Switching Frequency In order for

the switcher to maintain regulation, the energy that is transferred

to the inductor during the on-time must be transferred to the out-

put capacitor during the off-time. This relationship must be main-

tained for stable operation and governs the fundamental operation

of a switching regulator. Each component along the current path

changes the voltage across the inductor and therefore the energy

that is transferred during each cycle. Summing the voltage from

VIN to VOUT during each cycle gives a relationship of the voltage

across the inductor during the on-time and during the off-time.

These terms are represented as VON and VOFF .

Given a target operating frequency, represent tON as:

tON = T × D

(10)

where T equals 1 / fSW , and D is the duty cycle.

Duty cycle can be represented as the voltage across the inductor

during the off-time, divided by the total voltage of the off-time

and on-time:

D = VOFF / (VOFF + VON )

(11)

Next, determine the voltage drops during the on cycle and the

off cycle. Figure 2 shows the current path during the on-time and

off-time.

Creating voltage summation during each cycle will give equa-

tions to represent VON and VOFF :

VON = VIN – VOUT – (IOUT × RL ) – (RDS × IOUT ) (12)

VOFF = VOUT + (IOUT × RL ) + Vf + (RS × IOUT ) (13)

Now substituting VON and VOFF into equation 11 gives a com-

plete formula for duty cycle as it relates to the voltage across the

inductor:

D=

VOUT × (IOUT× RL ) + Vf + ( RS× IOUT)

VOUT + (IOUT× RL ) + Vf + ( RS× IOUT) +

VIN – VOUT – (IOUT× RL ) – ( RDS+ IOUT)

(14)

The effects of the voltage drop across the inductor resistance and

trace resistance do have an effect on the switching frequency.

However, the frequency variation due to these factors is small

and is covered in the variation of the switcher period, TSW , which

is ±25% of the target. Removing these current-dependent terms

simplifies the equation:

D=

VOUT + Vf + ( RS× IOUT)

VOUT + Vf + ( RS× IOUT) +

VIN – VOUT – ( RDS× IOUT)

(15)

Further simplification and grouping of terms yields:

D=

VOUT + Vf + ( RS× IOUT)

VIN + Vf + ( RS× IOUT) – ( RDS× IOUT)

(16)

Substitute this simplified expression for duty cycle back into

equation 10. The following formula results in the on-time, given

a target switching frequency:

1

tON = fSW

VOUT + Vf + ( RS× IOUT)

VIN + Vf + ( RS× IOUT) – ( RDS× IOUT)

(17)

The formulas above describe how tON changes based on input and

load conditions. Because load changes are minimal, and the out-

put voltage is fixed, the dominant factor that effects on-time is the

input voltage. The converter is able to maintain a constant period

over a varying supply voltage because the on-time is proportional

to the input voltage. The current into the TON terminal is derived

from a resistor tied to VIN1, which sets the on-time proportional

to the supply voltage. Selecting the resistor value, based on the

tON calculated above, is done using the following formula:

RTON

=

(tON

– 60 × 10 –9 )×

3.12 × 10 –12

VIN

(18)

After the resistor is selected and a suitable tON is found, it must

be demonstrated that tON does not, under worst-case conditions,

VIN1

VRL Current path (on-cycle)

LX

L1

A4402

Vf

VRL

Current path

VRS (off-cycle)

RLOAD

Star Ground

Figure 2. Current limiting during overload

Allegro MicroSystems, Inc.

11

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Share Link: