(

)

IN

OUT

OUT

OUT(LL)

SW

IN

V

V

V

1

I

=

2

L

V

-

´

´

´

´ f

TPS56428

www.ti.com

SLVSBV4B – APRIL 2013 – REVISED OCTOBER 2015

Feature Description (continued)

At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one

shot timer expires. This one shot is set by the converter input voltage, VIN, and the output voltage, VO, to

maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The

one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the

reference voltage. An internal ramp is added to reference voltage to simulate output ripple, eliminating the need

for ESR induced output ripple from D-CAP2™ mode control.

8.3.2 PWM Frequency and Adaptive On-Time Control

TPS56428 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The

TPS56428 runs with a pseudo-constant frequency of 650 kHz by using the input voltage and output voltage to

set the on-time one-shot timer. The on-time is inversely proportional to the input voltage and proportional to the

output voltage, therefore, when the duty ratio is VOUT/VIN, the frequency is constant.

8.3.3 Advanced Auto-Skip Eco-mode™ Control

The TPS56428 is designed with advanced auto-skip Eco-mode™ to increase higher light load efficiency. As the

output current decreases from heavy load condition, the inductor current is also reduced. If the output current is

reduced enough, the inductor current ripple valley reaches the zero level, which is the boundary between

continuous conduction and discontinuous conduction modes. The rectifying low-side MOSFET is turned off when

its zero inductor current is detected. As the load current further decreases the converter run into discontinuous

conduction mode. The on-time is kept approximately the same as is in continuous conduction mode. The off-time

increases as it takes more time to discharge the output capacitor to the level of the reference voltage with

Equation 1.

(1)

8.3.4 Soft Start and Pre-Biased Soft Start

The TPS56428 has an internal 1.0ms soft-start. When the EN pin becomes high, internal soft-start function

begins ramping up the reference voltage to the PWM comparator.

The TPS56428 contains a unique circuit to prevent current from being pulled from the output during startup if the

output is pre-biased. When the soft-start commands a voltage higher than the pre-bias level (internal soft start

the first low side FET gate driver pulses with a narrow on-time. It then increments that on-time on a cycle-by-

cycle basis until it coincides with the time dictated by (1-D), where D is the duty cycle of the converter. This

scheme prevents the initial sinking of the pre-bias output, and ensure that the out voltage (VO) starts and ramps

up smoothly into regulation and the control loop is given time to transition from pre-biased start-up to normal

mode operation.

8.4 Device Functional Modes

8.4.1 Power Good

The power-good function is activated after soft start has finished. The power good function becomes active after

1.7 times soft-start time. When the output voltage becomes within –10% of the target value, internal comparators

detect power good state and the power good signal becomes high. The power good output, PG is an open drain

output. If the feedback voltage goes under 15% of the target value, the power good signal becomes low.

8.4.2 Output Discharge Control

TPS56428 discharges the output via SW pin when EN is low, or the controller is turned off by the protection

functions(UVP, UVLO and thermal shutdown). The internal regular low-side MOSFET is not turned on during the

output discharge operation to avoid the possibility of causing negative voltage at the output

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