TPS65232 datasheet(16/31 Pages) TI1

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- V

OUT

L =

-V

IN(MAX)

OUT

0.3

· IOUT

¾ ·

OUT

IN(MAX)

¾ ·

L(RMS)

Ö(I

L(avg)

2 + 1

)

RIPPLE

) =Ö

)

OUT

2 + 1

12 (I

)

RIPPLE

TPS65232

SLVSA42 – FEBRUARY 2010

www.ti.com

We calculate a trip resistor value of 210 k

Ω. Place a 1-nF capacitor parallel to R14. Please note that typical FET

RDS(ON) is specified at 10 mΩ. Since we used RDSON,MAX, for setting the current limit, the actual current flowing

through the inductor with a nominal FET can be higher than the peak current of 7.4 A before the current limit

kicks in. Make sure that the chosen inductor has the correct peak current capabilities.

FEEDBACK LOOP DESIGN

TPS65232 loop compensation looks like a type-II compensation network because an internal zero-pole pair can

provide additional phase boost to stabilize this voltage mode control DC/DC controller. The internal zero is

located at 45 kHz and the pole is located at 240 kHz. Ideally, the best cross-over frequency is around 1/10th of

the switching frequency.

FEEDBACK DIVIDER (R11, R12)

Select R11 between 10 k

Ω and 100 kΩ. For this design select 22.1 kΩ. Next, R12 Is selected to produce the

desired output voltage when VFB = 0.8 V using the following formula:

(18)

VFB = 0.8 V and R11 = 22.1 KΩ for VOUT = 5.0 V, R12 = 4.22 kΩ.

Error Amplifier Pole-Zero Selection

The design guidelines for TPS65232 Buck1 loop compensation are as follows:

1. Place a compensation zero at 8 kHz to boost the phase margin at the anticipated cross-over frequency.

2. Set the value of R and C of this to zero: C16 = 1000 pF and R13 = 20 k

Ω.

3. Add an additional pole by making C17 = 100 pF. This pole is used to attenuate high frequency noise.

4. If VIN is 20 V - 24 V, make C17 = 200 pF.

BUCK2 DESIGN GUIDELINE

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNIT

VIN

Input supply voltage

4.75

VIN RIPPLE

Input voltage ripple

IOUT, BUCK1 = 3 A

VOUT

Output voltage

1.8

Line regulation

VIN = 3 V to 6 V

Load regulation

IOUT, BUCK1 = 0 A to 3 A

VOUT RIPPLE

Output ripple

IOUT, BUCK1 = 3 A

VTRANS

Transient deviation

IOUT, BUCK1 = 1.5 A to 3 A

IOUT

Output current

VIN = 3 V to 6 V

fSW

Switching frequency

1000

kHz

INDUCTOR SELECTION (L2)

The inductor is typically sized for < 30% peak-to-peak ripple current (IRIPPLE). Given this target ripple current, the

required inductor size is calculated by Equation 5.

(19)

Solving Equation 19 with VIN(MAX) = 6 V, an inductor value of 1.4 mH is obtained. A standard value of 2.2 mH is

selected, resulting in 0.37-A peak-to-peak ripple. The RMS current through the inductor is approximated by

Equation 6.

(20)

Using Equation 20, the maximum RMS current in the inductor is about 3.002 A.

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