Datasheet

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Thermal Shutdown
Vcom Buffer
Boost Converter Design Procedure
1. Duty Cycle : D  1 
Vin  
Vout
(5)
2. Maximum output current : I
out


Isw 
Vin  D
2  ƒs  L

 (1  D)
(6)
3. Peak switch current : I
swpeak

Vin  D
2  ƒs  L

I
out
1  D
(7)
Inductor Selection
TPS65150
SLVS576 – SEPTEMBER 2005
DETAILED DESCRIPTION (continued)
A thermal shutdown is implemented to prevent damage because of excessive heat and power dissipation.
Typically, the thermal shutdown threshold is 155 ° C . When this threshold is reached, the device enters shutdown.
The device can be enabled again by cycling the input voltage to GND.
The VCOM Buffer is a transconductance amplifier designed to drive capacitive loads. The IN pin is the input of
the VCOM buffer. If the VCOM buffer is not required for certain applications, it is possible to shut down the
VCOM buffer by connecting IN to ground, reducing the overall quiescent current. The VCOM buffer features a
soft start, avoiding a large voltage drop at Vs during startup. The VCOM buffer input, IN, cannot be pulled
dynamically to ground during operation.
The first step in the design procedure is to verify whether the maximum possible output current of the boost
converter supports the specific application requirements. A simple approach is to estimate the converter
efficiency, by taking the efficiency numbers from the provided efficiency curves, or use a worst case assumption
for the expected efficiency, e.g. 75%.
With
Isw = Converter switch current (minimum switch current limit = 2.0 A)
fs = Converter switching frequency (typical 1.2 MHz)
L = Selected inductor value
η = Estimated converter efficiency (use the number from the efficiency plots, or 0.75 as an estimation)
The peak switch current is the steady-state peak switch current that the integrated switch, inductor, and external
Schottky diode has to be able to handle. The calculation must be done for the minimum input voltage where the
peak switch current is highest. For the calculation of the maximum current delivered by the boost converter it
needs to be considered that the positive and negative charge pumps as well as the VCOM buffer run from the
output of the boost converter as well.
Several inductors work with the TPS65150. Especially with external compensation, the performance can be
adjusted to the specific application requirements. The main parameter for the inductor selection is the inductor
saturation current, which should be higher than the peak switch current as calculated previously with additional
margin to cover for heavy load transients. The alternative, more conservative approach, is to choose the inductor
with a saturation current at least as high as the typical switch current limit of 2.5 A. The second important
parameter is the inductor DC resistance. Usually the lower the DC resistance the higher the efficiency. It is
important to note that the inductor DC resistance is not the only parameter determining the efficiency. For a
boost converter, where the inductor is the energy storage element, the type and material of the inductor
influences the efficiency as well. Especially at high switching frequencies of 1.2 MHz, inductor core losses,
proximity effects, and skin effects become more important. Usually an inductor with a larger form factor gives
higher efficiency. The efficiency difference between different inductors can vary between 2% to 10%. For the
TPS65150, inductor values between 3.3 µH and 6.8 µH are a good choice, but other values can be used as well.
Possible inductors are shown in Table 1 .
17