IN,MIN

OUT

OUT,MAX

DS(on)

L

V

V

I

(R

+ R )

=

+

´

OUT

ON

IN

OUT

PFM

2

IN

OUT

IN

ON

OUT

V

t

420 ns

V

2

I

f

V

V

V

t

V

L

=

´

´

=

-

´

´

TPS62085, TPS62086, TPS62087

www.ti.com

SLVSB70A – OCTOBER 2013 – REVISED JUNE 2015

9.3 Feature Description

9.3.1 Power Save Mode

As the load current decreases, the TPS62085, TPS62086, and TPS62087 enter Power Save Mode operation.

During Power Save Mode, the converter operates with reduced switching frequency and with a minimum

quiescent current maintaining high efficiency. The power save mode occurs when the inductor current becomes

discontinuous. Power Save Mode is based on a fixed on-time architecture, as related in Equation 1. The

switching frequency over the whole load current range is also shown in Figure 1 for a typical application.

(1)

In Power Save Mode, the output voltage rises slightly above the nominal output voltage, as shown in Figure 8.

This effect is minimized by increasing the output capacitor or inductor value. The output voltage accuracy in PFM

operation is reflected in the electrical specification table and given for a 22-

μF output capacitor.

9.3.2 100% Duty Cycle Low Dropout Operation

The devices offer low input-to-output voltage difference by entering 100% duty cycle mode. In this mode, the

high-side MOSFET switch is constantly turned on and the low-side MOSFET is switched off. This is particularly

useful in battery powered applications to achieve the longest operation time by taking full advantage of the whole

battery voltage range. The minimum input voltage to maintain output regulation, depending on the load current

and output voltage can be calculated as:

with

•

•

•

•

(2)

9.3.3 Soft Start

The TPS62085, TPS62086, and TPS62087 have an internal soft-start circuitry which monotonically ramps up the

output voltage and reaches the nominal output voltage during a soft-start time of typically 0.8 ms. This avoids

excessive inrush current and creates a smooth output voltage slope. It also prevents excessive voltage drops of

primary cells and rechargeable batteries with high internal impedance. The device is able to start into a

prebiased output capacitor. The device starts with the applied bias voltage and ramps the output voltage to its

nominal value.

9.3.4 Switch Current Limit and Hiccup Short-Circuit Protection

The switch current limit prevents the devices from high inductor current and from drawing excessive current from

the battery or input voltage rail. Excessive current might occur with a shorted or saturated inductor or a heavy

is turned off and the low-side MOSFET is turned on to ramp down the inductor current. When this switch current

limits is triggered 32 times, the devices stop switching and enable the output discharge. The devices then

automatically start a new start-up after a typical delay time of 66 µs has passed. This is named HICCUP short-

circuit protection. The devices repeat this mode until the high load condition disappears.

9.3.5 Undervoltage Lockout

To avoid misoperation of the device at low input voltages, an undervoltage lockout (UVLO) is implemented,

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Product Folder Links: TPS62085 TPS62086 TPS62087