TPS61235P Datasheet, PDF (12/38 Pages) Texas Instruments – TPS6123x 8-A Valley Current Synchronous Boost Converters with Constant Current Output Feature

TPS61235P, TPS61236P
SLVSCK4A – SEPTEMBER 2015 – REVISED MAY 2016
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Feature Description (continued)
The CC pin can be connected to ground to disable the output current monitor function, and it will not affect the
CV operation.
8.3.4.3 Constant Current Operation
As Equation 1 shows, the CC pin voltage is proportional to the output current. The TPS6123x monitors the CC
pin voltage and compares it to an internal reference voltage V_REF, which is 1.244 V typically. When V_CC exceeds
V_REF, the CC regulation loop kicks in and pulls the inductor current valley to a lower value so to keep the CC pin
voltage at V_REF. Equally, the output current is regulated at the set value as Equation 2.
I_{OUT _ CC}
V_{REF ˜ K}
R_CC
(2)
Where:
I_{OUT_CC} is the set constant output current,
V_REF is the internal reference voltage, 1.244 V typically,
R_CC is the resistor connected at the CC pin,
K is the coefficient between the output current and the internal built current signal, K = 100,000.
If the load current is higher than the CC setting, the output voltage drops. A balance can be achieved if the load
decreases and matches the CC current before V_OUT is pulled below input voltage. In the balance status, the
TPS6123x can keep CC operation, output the constant current, and maintain the output voltage at the balanced
level. If the output voltage is pulled below the input voltage by a strong load before the balance is achieved, the
device exits CC operation and enters into start-up process, where the output current is limited by I_{LIM_pre} instead
of the CC value. If the load is still higher than I_{LIM_pre}, the device will be stuck in the pre-charge phase; otherwise,
the device can complete the pre-charge phase, but its output voltage will be pulled down again in the switching
phase due to the limited output current, so an oscillation may happen.
In order to avoid the potential oscillation, the CC operation is only recommended for pure resistive loads or load
devices with dynamic power management function. For a resistive load, its resistance should be higher than V_IN /
I_{OUT_CC}; for a load device with dynamic power management function, which can regulate its input voltage to a set
value, a higher set voltage than V_IN of the TPS6123x is suggested. By doing this, a balance can be achieved
before the output voltage is pulled below the input voltage, so to avoid the TPS6123x entering into the startup
process.
For effective CC operation, a capacitor must be connected in parallel with R_CC at CC pin, and the CC value
should be set lower than the maximum output capability of the converter; otherwise, the TPS6123x will trigger the
over current protection first and fail to regulate the output current. Refer to the Over Current Protection section
for details.
The CC operation can be disabled by shorting the CC pin to ground. By doing so, the CC loop is disabled, so the
TPS6123x works as a normal boost converter to regulate the output voltage, and its maximum output current
capability is decided by the internal current limit.
8.3.5 Over Current Protection
To protect the device from over load condition, an internal cycle-by-cycle current limit is implemented. Once the
inductor valley current reaches the internal current limit, the protection is triggered and it clamps the valley
current at the limit I_LIM until next cycle comes.
Figure 15 illustrates the valley current limit scheme. The average of the rectifier ripple current equals the output
current, I_OUT(DC). When the load current increases, the loop increases the valley current accordingly. If the valley
current is increased above I_LIM, the off-time will be extended until the valley drops to I_LIM. Then the next cycle
begins.
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