TPS51117
SLVS631B –DECEMBER 2005–REVISED SEPTEMBER 2009...................................................................................................................................... www.ti.com
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, RTON can be determined by Equation 8 and Equation 9,
V
OUT
1
ƒ
T
+
ON(max)
V
IN(min)
(8)
ǒTON(max) * 50 nsǓ
V
IN(min)
3
2
R
+
[W]
19 10*12
TON
ǒVOUT
Ǔ
) 150 mV
(9)
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.
ǒVIN(max)
Ǔ
ǒVIN(max)
Ǔ
* V
V
* V
V
OUT
OUT
IN(max)
OUT
OUT
3
1
L
+
+
IND
V
V
I
ƒ
I
ƒ
IN(max)
IND(ripple)
OUT(max)
(10)
For applications that require fast transient response with minimum VOUT overshoot, consider a smaller
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.
ǒVIN(max)
Ǔ
* V
V
V
OUT
IN(max)
OUT
TRIP
1
I
+
)
IND(peak)
R
L ƒ
V
DS(on)
(11)
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.
V
0.015
V
OUT
OUT
OUT(max)
ESR +
[
60 [mW]
I
0.75
I
ripple
(12)
5. Choose MOSFETs
Loss-less current sensing and overcurrent protection of the TPS51117 is determined by RDS(on) of the
low-side MOSFET. So, RDS(on) times the inductor current value at the overcurrent point should be in the
range of 30 mV to 200 mV for the entire operational temperature range. Assuming a 20% guard band, RDS(on)
in the following equation should satisfy the full temperature range.
30 mV
* 0.5 I
200 mV
* 0.5 I
v R
v
DS(on)
1.2 I
1.2 I
OUT(max)
6. Choose Rtrip
Once the low-side FET is decided, select an appropriate Rtrip value that provides Vtrip equal to RDS(on) times
ripple
OUT(max)
ripple
(13)
Ipeak
.
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
18
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