Datasheet

N out
swpeak
V D I
I = +
2 s L 1 D
´
´ ¦ ´ -
I
( )
IN
V D
Iout = 1 D Isw
2 s L
æ ö
´
- -
ç ÷
´ ¦ ´
è ø
NEG
IN NEG
V
D =
V η + V´
IN out
swpeak
V D I
I = +
2 s L 1 D
´
´ ¦ ´ -
( )
IN
V D
Iout = 1 D Isw
2 s L
æ ö
´
- -
ç ÷
´ ¦ ´
è ø
TPS65137
www.ti.com SLVS929A –MAY 2010–REVISED OCTOBER 2012
(2)
The maximum output current is given at the highest switching frequency of typically 1.4MHz and minimum switch
current limit of 0.9A. Equation 3 is used to calculate the switch peak current.
(3)
The inductor needs to be rated for this switch peak current to avoid inductor saturation.
The boost converter output capacitor is connected to pin CB and a 4.7µF capacitor is sufficient. A 2.2µF
capacitor is used on the output V
POS
, which is the output of the internal low dropout regulator (LDO).
Table 3. Output Capacitor Selection
CAPACITOR COMPONENT SUPPLIER SIZE
4.7 μF/10 V Taiyo Yuden LMK107BJ475 0603
2.2 μF/10 V Taiyo Yuden LMK107BJ225 0603
NEGATIVE BUCK BOOST CONVERTER DESIGN CONSIDERATION, V
neg
The negative output is generated with a buck boost converter. The component values and output current are
calculated at maximum load current in continuous conduction operation. The typical switching frequency during
this operation mode is 1.4MHz.
The buck boost converter duty cycle is:
(4)
To calculate the duty cycle a good estimation for the efficiency, η, is 75% or it can be taken out of the typical
curve in Figure 1. In order to calculate the maximum output current of the buck boost converter for a certain input
voltage, the following formula is used:
(5)
The maximum output current is given at the highest switching frequency of typically 1.4MHz and minimum switch
current limit of 1.1A. Equation 6 is used to calculate the switch peak current.
(6)
The inductor needs to be rated for this switch peak current to avoid inductor saturation. Refer to Table 4 for
possible inductors for this application. A 4.7μF output capacitor is used on the output V
NEG
. Larger capacitor
values can be used to minimize the output voltage ripple. Refer to Table 3 for output capacitor selection.
INDUCTOR SELECTION
The device is optimized to operate with 4.7uH inductors. Different inductor values will change the converter
efficiency and output voltage ripple. A 2.2uH inductor is also a possible solution. Any other inductor values will
degrade device performance and stability which is not recommended for this device.
Table 4. Inductor Selection
INDUCTOR VALUE COMPONENT DIMENSIONS in mm Isat/DCR
SUPPLIER
4.7 μH TDK VLF4012 3.7 × 3.5 × 1.2 1.1A/140mΩ
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