SC415_08 datasheet(16/30 Pages) SEMTECH

16 / 30 page

SC415

During the DH on-time, voltage across the inductor is

(VIN - VOUT). To determine the inductance, the ripple

current must be defined.

Smaller ripple current will

give smaller output ripple voltage but will require larger

inductors. The ripple current will also set the boundary

for PSAVE operation. The switcher will typically enter

PSAVE operation when the load current decreases to ½ of

the ripple current; (i.e. if ripple current is 4A then PSAVE

operation will typically start for loads less than 2A. If ripple

current is set at 40% of maximum load current, then PSAVE

will occur for loads less than 20% of maximum).

The equation for inductance is:

L = (VIN - VOUT) × TON / I

RIPPLE

Use the maximum value for VIN, and for TON use the value

associated with maximum VIN, and that side’s TON using

the RTON value selected. For selecting the inductor, we

start with the highest VOUT setting and a maximum ripple

current of 4A.

TON1 = 343 nsec at 20VIN, 1.8VOUT

L = (20 - 1.8) × 343 nsec / 4A = 1.56μH

We will use 1.5μH which will slightly increase the maximum

IRIPPLE to 4.2A.

Note: the inductor must be rated for the maximum DC

load current plus ½ of the ripple current.

The minimum ripple current is also calculated. This occurs

when VIN is at the minimum value of 10V.

TON

VINMIN

= 3.3 × (RTON+37) × (VOUT/VIN) + 35

TON

VINMIN

= 651nsec

RIPPLE

= (VIN - VOUT) × TON / L

RIPPLE_VINMIN

= (10 – 1.8) × 651 nsec / 1.5μH

= 3.55A

Applications Information (continued)

Capacitor Selection

The output capacitors are chosen based on required ESR

and capacitance. The ESR requirement is driven by the

output ripple requirement and the DC tolerance. The

output voltage has a DC value that is equal to the valley

of the output ripple, plus ½ of the peak-to-peak ripple.

Changing the ripple voltage will lead to a change in DC

output voltage.

The design goal is +/-4% output regulation. The internal

750mV reference tolerance is 1%, and assuming 1%

tolerance for the FB resistor divider, this allows 2%

tolerance due to VOUT ripple. Since this 2% error comes

from ½ of the ripple voltage, the allowable ripple is 4%, or

72mV for a 1.8V output. Although this is acceptable from

a regulation standpoint, 72mV ripple is high for a 1.8V

output and therefore more realistic ripple value of 36mV

will be used (2% of VOUT).

The maximum ripple current of 4.2A creates a ripple

voltage across the ESR. The maximum ESR value allowed

would create 36mV ripple:

ESRMAX = V

RIPPLE

RIPPLEMAX

= 36mV / 4.2A

ESRMAX = 8.6 mΩ

While the ESR is chosen to meet ripple requirements,

the output capacitance (μF) is typically chosen based on

transient requirements. A worst-case load release, from

maximum load to no load at the exact moment when

inductor current is at the peak, defines the required

capacitance. If the load release is instantaneous (load

changes from maximum to zero in a very small time), the

output capacitor must absorb all the inductor’s stored

energy. This will cause a peak voltage on the capacitor

according to the equation:

OUTMIN

= L × (IOUT + 1/2 × I

RIPPLEMAX

_______________________

PEAK

2 - V

OUT

SC415 数据表(PDF) 16 Page - Semtech Corporation

SC415 数据表(HTML) 16 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

数据表下载

链接网址

SC415 数据表(PDF) 16 Page - Semtech Corporation

SC415 数据表(HTML) 16 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

数据表 下载

链接网址

数据表下载