TPS652510
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SLVSB30B – SEPTEMBER 2011 – REVISED AUGUST 2013
Output Inductor Selection
To calculate the value of the output inductor, use Equation 2.
Lo = Vin -Vout × Vout
Io × Kind Vin × fsw
(2)
Kind is a coefficient that represents the amount of inductor ripple current relative to the maximum output current.
In general, Kind is normally from 0.1 to 0.3 for the majority of applications. A value of 0.1 will improve the
efficiency at light load, while a value of 0.3 will provide the lowest possible cost solution. The ripple current is:
Iripple = Vin -Vout × Vout
Lo Vin × fsw
(3)
Output Capacitor
There are two primary considerations for selecting the value of the output capacitor. The output capacitors are
selected to meet load transient and output ripple’s requirements. If a minimum transient specification is required
use the following equation:
Co > DIOUT 2 × Lo
Vout × DVout
(4)
The following equation calculates the minimum output capacitance needed to meet the output voltage ripple
specification.
1
1
Co
>
8×
×
fsw
VRIPPLE
I RIPPLE
(5)
Where fSW is the switching frequency, VRIPPLE is the maximum allowable output voltage ripple, and IRIPPLE is the
inductor ripple current.
Input Capacitor
A minimum 10-µF X7R/X5R ceramic input capacitor is recommended to be added between VIN and GND of
each converter. The input capacitor must handle the RMS ripple current shown in the following equation.
Vout (Vin min-Vout )
Icirms = Iout ×
×
Vin min Vin min
(6)
Bootstrap Capacitor
The device has two integrated boot regulators and requires a small ceramic capacitor between the BST and LX
pins to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor should be
0.047 µF. A ceramic capacitor with an X7R or X5R grade dielectric is recommended because of the stable
characteristics over temperature and voltage.
Soft-Start Time
The device has an internal pull-up current source of 5 µA that charges an external soft-start capacitor to
implement a slow start time. Equation 7 shows how to select a soft-start capacitor based on an expected slow
start time. The voltage reference (VREF) is 0.8 V and the soft-start charge current (Iss) is 5 µA. The soft-start
circuit requires 1 nF per around 167 µs to be connected at the SS pin. A 0.8-ms soft-start time is implemented for
all converters fitting 4.7 nF to the relevant SS pin.
( ) Tss(ms) = VREF(V) · - CIssss((µnAF))
(7)
The Power Good circuit for the bucks has a 10-ms watchdog. Therefore the soft-start time should be lower than
this value. It is recommended not to exceed 5 ms.
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