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 ripple generation

circuit is added to reference voltage to emulate the output ripple, enabling the use of very low-ESR output

capacitors such as multi-layered ceramic caps (MLCC). No external current sense network or loop compensation

is required for D-CAP3 control mode.

7.3.2 Eco-mode Control

The TPS563252 is designed with advanced Eco-mode to maintain high light-load efficiency. As the output

current decreases from heavy load condition, the inductor current is also reduced and eventually comes to point

that its ripple valley touches zero level, which is the boundary between continuous conduction and discontinuous

conduction mode. The rectifying MOSFET is turned off when the zero inductor current is detected. As the load

current further decreases, the converter runs into discontinuous conduction mode. The on time is kept almost the

same as it was in continuous conduction mode so that it takes longer time to discharge the output capacitor with

smaller load current to the level of the reference voltage. This makes the switching frequency lower, proportional

to the load current, and keeps the light load efficiency high. Use the below equation to calculate the transition

point to the light load operation IOUT(LL) current.

IN

OUT

OUT

OUT(LL)

SW

IN

(V

V

)

V

1

I

2

L

f

V

u

u

u

u

(1)

7.3.3 Soft Start and Prebiased Soft Start

The TPS56325x has an internal fixed 1.6-ms soft-start time. The EN default status is low. When the EN pin

becomes high, the internal soft-start function begins ramping up the reference voltage to the PWM comparator.

If the output capacitor is prebiased at start-up, the devices initiate switching and start ramping up only after the

internal reference voltage becomes higher than the feedback voltage, VFB. This scheme makes sure that the

converter ramps up smoothly into the regulation point.

7.3.4 Overvoltage Protection

The TPS56325x has the overvoltage protection feature. When the output voltage becomes higher than the

OVP threshold, the OVP is triggered with a 24-μs deglitch time. Both the high-side MOSFET and the low-side

MOSFET drivers are turned off. When the overvoltage condition is removed, the device returns to switching.

7.3.5 Large Duty Operation

The TPS56325x can support large duty operations up to 95% by smoothly dropping down the switching

frequency. When VIN / VOUT < 1.6 and VFB is lower than internal VREF, the switching frequency is allowed to

smoothly drop to make tON extended to implement the large duty operation and improve the performance of the

load transient. Please refer frequency test waveform in Figure 6-8. The minimum switching frequency is limited

to approximately 450 kHz.

7.3.6 Current Protection and Undervoltage Protection

The output overcurrent limit (OCL) is implemented using a cycle-by-cycle valley detect control circuit. The switch

current is monitored during the off state by measuring the low-side FET drain-to-source voltage. This voltage is

proportional to the switch current. To improve accuracy, the voltage sensing is temperature compensated.

During the on time of the high-side FET switch, the switch current increases at a linear rate determined by the

following:

• VIN

• VOUT

• On-time

• Output inductor value

During the on time of the low-side FET switch, this current decreases linearly. The average value of the switch

current is the load current, IOUT. If the monitored valley current is above the OCL level, the converter maintains a

low-side FET on and delays the creation of a new set pulse, even the voltage feedback loop requires one, until

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TPS563252, TPS563257

SLUSEQ5A – AUGUST 2022 – REVISED MAY 2023

Copyright © 2023 Texas Instruments Incorporated

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