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TPS61236 Datasheet(PDF) 11 Page - Texas Instruments

No. de Pieza. TPS61236
Descripción  TPS6123x 8-A Valley Current Synchronous Boost Converters with Constant Current Output Feature
Descarga  38 Pages
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Fabricante  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
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TPS61236 Datasheet(HTML) 11 Page - Texas Instruments

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CC
OUT
CC
R
K
I
V
˜
11
TPS61235P, TPS61236P
www.ti.com
SLVSCK4A – SEPTEMBER 2015 – REVISED MAY 2016
Product Folder Links: TPS61235P TPS61236P
Submit Documentation Feedback
Copyright © 2015–2016, Texas Instruments Incorporated
Feature Description (continued)
Once the output capacitor is charged close to the input voltage, the converter starts switching. This is called the
start-up switching phase. During the phase, the converter steps up the voltage to its nominal output voltage by
following an internal ramp up reference voltage, which ramps up in around 3 ms (typ.) to its final value. The
current limit function is activated in this phase.
Because of the current limitation during the pre-charge phase, the TPS6123x may not be able to start up under a
heavy load condition. It is recommended to apply no load or a light load during the startup process, and apply the
full load only after the TPS6123x starts up successfully. Refer to Figure 8 for the recommended minimum load
resistance.
8.3.3 Enable and Disable
An external logic signal at the EN pin can enable and disable the device.
The TPS6123x device starts operation when EN is set high and starts up with the soft-start process. For proper
operation, the EN pin must be terminated and must not be left floating. Pulling EN low forces the device into
shutdown, with a shutdown current of typically 0.01 µA. In shutdown mode, a true disconnection between input
and output is implemented. It can prevent current from input to output, or reverse current from output to input.
8.3.4 Constant Output Voltage and Constant Output Current Operations
Normally a boost converter only regulates its output voltage, but for the TPS6123x, it is different. There are two
regulation loops for the device. One loop regulates the output voltage, and it is called CV (Constant Voltage)
operation; the other regulates the output current, and it is called CC (Constant Current) operation.
8.3.4.1 Constant Voltage Operation
Before the output current reaches the constant current value programmed by an external resistor at the CC pin,
the voltage regulation loop dominates. The output voltage is monitored via external or internal feedback network
resistors at the FB pin. An error amplifier compares the feedback voltage to an internal reference voltage VREF
and adjusts the inductor current valley accordingly. In this way, the TPS6123x operates as a normal boost
converter to regulate the output voltage.
During CV operation, the maximum VIN should be 0.6-V below VOUT to keep the output voltage well regulated.
The TPS6123x may enter into pass-through operation prematurely when VIN is close to but still below VOUT, and
exists when VIN is below the threshold with a hysteresis. When in pass-through operation, the boost converter
stops switching and keeps the rectifying switch on, so the input voltage can pass through the external inductor
and internal rectifying switch to the output. The output current capability becomes lower and is limited by the pre-
charge current limit ILIM_pre of the rectifying switch. More than 0.4-V under-voltage of VOUT may occur due to the
premature pass-through operation and the hysteresis of existing. If the under-voltage is not acceptable, the
maximum VIN should be limited to 0.6-V below VOUT , which gives enough margin to avoid the pass-through
operation.
8.3.4.2 Output Current Monitor
During the CV operation, the output current can be monitored at the CC pin. In the TPS6123x, the inductor
current is sensed through the rectifying switch during the off-time of each switching cycle. The device then builds
a current signal which is 1/K times the sensed current and feeds it to the CC pin during off-time. As a result, the
CC pin voltage, VCC, is proportional to the average output current as Equation 1 shows.
(1)
Where:
VCC is the voltage at the CC pin,
IOUT is the output current,
K is the coefficient between the output current and the internal built current signal, K = 100,000,
RCC is the resistor connected at the CC pin.
A capacitor must be connected in parallel with RCC to average the CC pin voltage and also stabilize the control
loop. Normally a 10-nF capacitor is recommended. A larger capacitor at the CC pin will smooth the CC voltage
better, and also slow down the loop response.


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