TPS53317 datasheet(21/35 Pages) TI1

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%176:IEJ ;,75 =

+176

. ×

8176

8+0

P59 + P/+0:KBB;A

2 ×

8176 × 875 ×

8+0 . 8176

8+0

P59 . P/+0:KBB;A

176

:IEJ ;,15 =

:3 A;2 × 0.25 µ*

2 × 0.6 8 × 0.03 8

= 62.5 µ(

176

:IEJ ;,15 =

+176

× .

2 × 8176 × 815

(800 G*V × 1.25 #)

× 0.68 × 0.45 = 0.270 µ*

B59 × +2.2

× 8176 × (1 . &)

+2.2 = 2.5 # × 0.5 = 1.25 #

TPS53317

www.ti.com

SLUSAK4D – JUNE 2011 – REVISED JULY 2015

Typical Applications (continued)

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Step 1. Determine Configuration

Because this DDR4 application requires a tight load tolerance, fast transient response, and sinking current

capability, the design uses a non-droop PWM configuration. Choose 600-kHz switching frequency due to the

duty cycle and minimim off-time of the device, and set an overcurrent (OC) valley limit of 5.4 A due to the

maximum load requirement of 2.5 A. Referring to Table 1 select an RMODE value of 68 kΩ.

8.2.1.2.2

Step 2. Select Inductor

Smaller inductor values have better transient performance but higher ripple and lower efficiency. High values

have the opposite characteristics. It is common practice to limit the ripple current to 30% to 50% of the maximum

current. Choose 50% to allow use of a smaller inductor for faster transient performance.

(7)

where

•

D = duty cycle

(8)

Because this device operates in DCAP+ mode, the frequency and duty cycle vary based on the input voltage, the

output voltage and load. With a 2.5-A load, a 1.2-V input voltage and 0.60 V output voltage, fSW is experimentally

measured at approximately 800 kHz and duty cycle of 0.55. Therefore L is calculated as shown in Equation 10.

(9)

Choose the closest standard value, 0.25 µH.

8.2.1.2.3

Step 3. Determine Output Capacitance

Use Equation 10 to calculate the output capacitance for a desired maximum overshoot.

where

•

COUT(min),OS is the minimum output capacitance for a desired overshoot

•

ΔIOUT is the maximum output current change in the application

•

VOUT = desired output voltage

•

VOS is the desired output voltage change due to overshoot

(10)

Choose a value of 30 mV to account for normal output voltage ripple.

(11)

Use Equation 12 to calculate the necessary output capacitance for a desired maximum undershoot.

where

•

COUT(min),US is the minimum output capacitance for a desired undershoot

•

VUS is the desired output voltage change due to overshoot

•

tSW is the period of switch node

•

tMIN(off) is the minimum off-time (270 ns)

(12)

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