Datasheet

load current
Output voltage
Vout (PWM)
Vout +1%
PFM Comp
PFM Mode
Vout -1%
PFM Comp Low
PWM MODE
Voltage Positioning
TPS65217A, TPS65217B, TPS65217C, TPS65217D
www.ti.com SLVSB64F –NOVEMBER 2011–REVISED APRIL 2013
During the Power Save Mode the output voltage is monitored with a PFM comparator. As the output voltage falls
below the PFM comparator threshold of V
OUT
- 1%, the device starts a PFM current pulse. For this the high-side
MOSFET will turn on and the inductor current ramps up. Then it is turned off and the low-side MOSFET switch
turns on until the inductor current becomes 0 again.
The converter effectively delivers a current to the output capacitor and the load. If the load is below the delivered
current the output voltage will rise. If the output voltage is equal or higher than the PFM comparator threshold,
the device stops switching and enters a sleep mode with typically 15-µA current consumption. In case the output
voltage is still below the PFM comparator threshold, further PFM current pulses will be generated until the PFM
comparator threshold is reached. The converter starts switching again once the output voltage drops below the
PFM comparator threshold.
With a single threshold comparator, the output voltage ripple during PFM mode operation can be kept very small.
The ripple voltage depends on the PFM comparator delay, the size of the output capacitor and the inductor
value. Increasing output capacitor values and/or inductor values will minimize the output ripple.
The PFM mode is left and PWM mode entered in case the output current can no longer be supported in PFM
mode or if the output voltage falls below a second threshold, called PFM comparator low threshold. This PFM
comparator low threshold is set to -1% below nominal V
OUT
, and enables a fast transition from Power Save Mode
to PWM Mode during a load step.
The Power Save Mode can be disabled through the I
2
C interface for each of the step-down converters
independent from each other. If Power Save Mode is disabled, the converter will then operate in fixed PWM
mode.
Dynamic Voltage Positioning
This feature reduces the voltage under/overshoots at load steps from light to heavy load and vice versa. It is
active in Power Save Mode. It provides more headroom for both the voltage drop at a load step, and the voltage
increase at a load throw-off. This improves load transient behavior. At light loads, in which the converter operates
in PFM mode, the output voltage is regulated typically 1% higher than the nominal value. In case of a load
transient from light load to heavy load, the output voltage drops until it reaches the PFM comparator low
threshold set to –1% below the nominal value and enters PWM mode. During a load throw off from heavy load to
light load, the voltage overshoot is also minimized due to active regulation turning on the low-side MOSFET.
Figure 15. Dynamic Voltage Positioning in Power Save Mode
100% Duty Cycle Low Dropout Operation
The device starts to enter 100% duty cycle Mode once the input voltage comes close the nominal output voltage.
In order to maintain the output voltage, the high-side MOSFET is turned on 100% for one or more cycles. As VIN
decreases further, the high-side MOSFET is turned on completely. In this case the converter offers a low input-
to-output voltage difference. This is particularly useful in battery-powered applications to achieve longest
operation time by taking full advantage of the whole battery voltage range.
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