Datasheet

2
maxmax
L
outL
I
II
D
+=
fL
Vin
Vout
VoutI
L
×
-
×=D
1
95%
5%
t
RAMP
t
Start
EN
V
OUT
( )
LMAXDSONMAXOUTMAXOUTMININ
RRIVV +×+=
,,,,
TPS65217A, TPS65217B, TPS65217C, TPS65217D
SLVSB64F –NOVEMBER 2011–REVISED APRIL 2013 www.ti.com
The minimum input voltage to maintain regulation depends on the load current and output voltage, and can be
calculated as:
(2)
where:
I
OUT,MAX
= Maximum output current plus inductor ripple current
R
DSON,MAX
= Maximum upper MOSFETt switch R
DSON
R
L
= DC resistance of the inductor
V
OUT,MAX
= Nominal output voltage plus maximum output voltage tolerance
Short-Circuit Protection
High-side and low-side MOSFET switches are short-circuit protected. Once the high-side MOSFET switch
reaches its current limit, it is turned off and the low-sideMOSFET switch is turned ON. The high-side MOSFET
switch can only turn on again, once the current in the low-sideMOSFET switch decreases below its current limit.
Soft Start
The 3 step-down converters in TPS65217 have an internal soft start circuit that controls the ramp up of the
output voltage. The output voltage ramps up from 5% to 95% of its nominal value within 750 µs. This limits the
inrush current in the converter during start up and prevents possible input voltage drops when a battery or high
impedance power source is used. The soft start circuit is enabled after the start up time t
Start
has expired.
Figure 16. Output of the DCDC Converters is Ramped Up Within 750 µs
Output Filter Design (Inductor and Output Capacitor)
Inductor Selection for Buck Converters
The step-down converters operate typically with 2.2-µH output inductors. Larger or smaller inductor values can
be used to optimize the performance of the device for specific operation conditions. The selected inductor has to
be rated for its DC resistance and saturation current. The DC resistance of the inductance will influence directly
the efficiency of the converter. Therefore an inductor with lowest DC resistance should be selected for highest
efficiency.
The following formula can be used to calculate the maximum inductor current under static load conditions. The
saturation current of the inductor should be rated higher than the maximum inductor current because during
heavy load transient the inductor current will rise above the calculated value.
(3)
(4)
32 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: TPS65217A TPS65217B TPS65217C TPS65217D