Datasheet
L
L_peak L_DC
L_DC
L
SW
I
I = I +
2
Vout Iout
I =
Vin
1
I =
1 1
L +
Vout + 0.8 V VIN VIN
D
´
´ h
D
é ù
æ ö
´ ¦ ´
ê ú
ç ÷
-
è ø
ë û
out
out
ripple
D I
C =
Fs V
´
´
TPS61093
SLVS992A –SEPTEMBER 2009–REVISED NOVEMBER 2009
www.ti.com
(5)
Where
I
L_peak
= Peak switch current
I
L_DC
= Inductor average current
ΔI
L
= Inductor peak to peak current
η = Estimated converter efficiency
Normally, it is advisable to work with an inductor peak-to-peak current of less than 30% of the average inductor
current. A smaller ripple from a larger valued inductor reduces the magnetic hysteresis losses in the inductor and
EMI. But in the same way, load transient response time is increased. Also, the inductor value should not be
outside the 2.2-μH to 10-μH range in the recommended operating conditions table. Otherwise, the internal slope
compensation and loop compensation components are unable to maintain small signal control loop stability over
the entire load range. Table 1 lists the recommended inductor for the TPS61093.
Table 1. Recommended Inductors for the TPS61093
L DCR MAX SATURATION
PART NUMBER SIZE (L×W×H mm) VENDOR
(μH) (mΩ) CURRENT (A)
#A915_Y-4R7M 4.7 45 1.5 5.2x5.2x3.0 Toko
#A915_Y-100M 10 90 1.09 5.2x5.2x3.0 Toko
VLS4012-4R7M 4.7 132 1.1 4.0x4.0x1.2 TDK
VLS4012-100M 10 240 0.82 4.0x4.0x1.2 TDK
CDRH3D23/HP 10 198 1.02 4.0x4.0x2.5 Sumida
LPS5030-103ML 10 127 1.4 5.0x5.0x3.0 Coilcraft
INPUT AND OUTPUT CAPACITOR SELECTION
The output capacitor is mainly selected to meet the requirements for output ripple and loop stability. This ripple
voltage is related to the capacitor’s capacitance and its equivalent series resistance (ESR). Assuming a ceramic
capacitor with zero ESR, the minimum capacitance needed for a given ripple can be calculated by:
(6)
Where V
ripple
= peak to peak output ripple. The ESR impact on the output ripple must be considered if tantalum
or electrolytic capacitors are used.
Care must be taken when evaluating a ceramic capacitor’s derating under dc bias, aging, and ac signal. For
example, larger form factor capacitors (in 1206 size) have their self resonant frequencies in the range of the
switching frequency. So the effective capacitance is significantly lower. The dc bias can also significantly reduce
capacitance. A ceramic capacitor can lose as much as 50% of its capacitance at its rated voltage. Therefore,
always leave margin on the voltage rating to ensure adequate capacitance at the required output voltage.
A 4.7-μF (minimum) input capacitor is recommended. The output requires a capacitor in the range of 1 μF to 10
μF. The output capacitor affects the small signal control loop stability of the boost regulator. If the output
capacitor is below the range, the boost regulator can potentially become unstable.
The popular vendors for high value ceramic capacitors are:
• TDK (http://www.component.tdk.com/components.php)
• Murata (http://www.murata.com/cap/index.html)
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