www.ti.com

T

ON(max) *

1

ƒ

V

OUT

V

IN(min)

(8)

R

TON

3

2

T

ON(max) [

50 ns

19

V

IN(min)

V

OUT *

150 mV

[q]

(9)

L

IND [

1

I

IND(ripple)

ƒ

V

IN(max) *

V

OUT

V

OUT

V

IN(max)

[

3

I

OUT(max)

ƒ

V

IN(max) *

V

OUT

V

OUT

V

IN(max)

(10)

I

IND(peak)

V

TRIP

R

DS(on)

*

1

L

ƒ

V

IN(max) [

V

OUT

V

OUT

V

IN(max)

(11)

ESR *

V

OUT

0.015

I

ripple

0.75 [

V

OUT

I

OUT(max)

60 [mq]

(12)

30 mV

1.2

I

OUT(max) *

0.5

I

ripple

[ RDS(on) [

200 mV

1.2

I

OUT(max) *

0.5

I

ripple

(13)

TPS51117

SLVS631 – DECEMBER 2005

APPLICATION INFORMATION (continued)

Switching frequency is usually determined by the overall view of the DC-DC converter design of: size,

efficiency or cost, and mostly dictated by external component constraints such as the size of inductor and/or

output capacitor. In the case where an extremely low or high duty factor is expected, the minimum on-time or

off-time also needs to be considered to satisfy the required duty factor. Once the switching frequency is

decided, R

3. Choose inductor

A good starting point inductance value is where the ripple current is approximately 1/4 to 1/2 of the maximum

output current.

For applications that require fast transient response with minimum V

inductance than above. The cost of a small inductance value is higher steady state ripple, larger line

regulation, and higher switching loss.

The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak

inductor current before saturation. The peak inductor current can be estimated as follows.

4. Choose output capacitor(s)

Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. Determine ESR to

meet the required ripple voltage above. A quick approximation is shown in Equation 12.

5. Choose MOSFETs

Loss-less current sensing and overcurrent protection of the TPS51117 is determined by R

low-side MOSFET. So, R

range of 30 mV to 200 mV for the entire operational temperature range. Assuming a 20% guard band, R

DS(on)

in the following equation should satisfy the full temperature range.

6. Choose R

trip

Once the low-side FET is decided, select an appropriate R

I

7. LPF for V5FILT

In order to reject high frequency noise and also secure safe start-up of the internal reference circuit, apply

1

µF of MLCC closely at the V5FILT pin with a 300-Ω resistor to create a LPF between +5-V supply and the

pin.

8. VBST capacitor, VBST diode

17