TPS62590

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SLVS897C – JANUARY 2009 – REVISED DECEMBER 2015

7.4 Device Functional Modes

7.4.1 Soft-Start

The TPS62590 has an internal soft-start circuit that controls the ramp up of the output voltage. The output

voltage ramps up from 5% to 95% of its nominal value within typical 250

μs. This limits the inrush current in the

converter during ramp up and prevents possible input voltage drops when a battery or high impedance power

7.4.2 Power-Save Mode

The power-save mode is enabled with MODE pin set to low level. If the load current decreases, the converter will

enter power save mode operation automatically. During power save mode the converter skips switching and

operates with reduced frequency in PFM mode with a minimum quiescent current to maintain high efficiency. The

converter will position the output voltage +1% above the nominal output voltage typically. This voltage positioning

feature minimizes voltage drops caused by a sudden load step.

The transition from PWM mode to PFM mode occurs once the inductor current in the low-side MOSFET switch

becomes zero, which indicates discontinuous conduction mode.

During the power-save mode the output voltage is monitored with a PFM comparator. As the output voltage falls

high-side MOSFET switch will turn on and the inductor current ramps up. After the ON-time expires, the switch is

turned off and the low-side MOSFET switch is turned on until the inductor current becomes zero.

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 to or higher than the PFM comparator threshold,

the device stops switching and enters a sleep mode with typical 15-

μA current consumption.

If the output voltage is still below the PFM comparator threshold, a sequence of further PFM current pulses are

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 fast single threshold comparator, the output voltage ripple during PFM mode operation can be kept small.

The PFM pulse is time controlled, which allows to modify the charge transferred to the output capacitor by the

value of the inductor. The resulting PFM output voltage ripple and PFM frequency depend in first order on the

size of the output capacitor and the inductor value. Increasing output capacitor values and inductor values will

minimize the output ripple. The PFM frequency decreases with smaller inductor values and increases with larger

values.

The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM

mode. The power save mode can be disabled through the MODE pin set to high. The converter will then operate

in fixed frequency PWM mode.

7.4.3 100% Duty Cycle Low Dropout Operation

The device starts to enter 100% duty cycle mode once the input voltage comes close the nominal output voltage.

To maintain the output voltage, the high-side MOSFET switch is turned on 100% for one or more cycles.

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.

The minimum input voltage to maintain regulation depends on the load current and output voltage, and can be

calculated by Equation 1:

where

•

•

•

•

(1)

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