C6(nF) x V

C6(nF) x 0.765 x 1.1

REF

t

(ms) =

=

SS

I

( A)

2

SS

x 1.1

m

(

)

(

)

2

IN

OUT

OUT

OUT LL

IN

V

V

V

I

L fsw

V

-

×

=

×

× ×

1

9

TPS54228

www.ti.com

SLVSAU1E – MAY 2011 – REVISED AUGUST 2016

Product Folder Links: TPS54228

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Copyright © 2011–2016, Texas Instruments Incorporated

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.

7.3.2 PWM Frequency and Adaptive On-Time Control

TPS54228 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The

TPS54228 runs with a pseudo-constant frequency of 700 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.

7.3.3 Auto-Skip Eco-Mode™ Control

The TPS54228 is designed with Auto-Skip Eco-Mode™ to increase 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

rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous

conduction modes. The rectifying 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 almost the

same as is was in the 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. The transition point to the light load operation

(1)

7.3.4 Soft Start and Prebiased Soft Start

The soft-start function is adjustable. When the EN pin becomes high, 2-µA current begins charging the capacitor

which is connected from the SS pin to GND. Smooth control of the output voltage is maintained during start-up.

The equation for the slow start time is Equation 2. VFB voltage is 0.765 V and SS pin source current is 2 µA.

(2)

The TPS54228 contains a unique circuit to prevent current from being pulled from the output during start-up if the

output is prebiased. When the soft start commands a voltage higher than the prebias 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

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

mode operation.

7.3.5 Current Protection

The output overcurrent protection (OCP) is implemented using a cycle-by-cycle valley detect control circuit. The

switch current is monitored by measuring the low-side FET switch voltage between the SW pin and GND. This

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

compensated.

monitors the low-side FET switch voltage, which is proportional to the switch current, during the low-side on-time.

If the measured voltage is above the voltage proportional to the current limit, an internal counter is incremented

per each SW cycle and the converter maintains the low-side switch on until the measured voltage is below the

voltage corresponding to the current limit at which time the switching cycle is terminated and a new switching