t

ON

t

CLdelay

CLPeak

Peak Inductor Current

Open Loop Current Limit

/L x t

IN

CLdelay

TPS54341

www.ti.com

SLVSC61 – NOVEMBER 2013

DETAILED DESCRIPTION (continued)

Accurate Current Limit Operation and Maximum Switching Frequency

The TPS54341 device implements peak-current-mode control in which the COMP pin voltage controls the peak

current of the high-side MOSFET. A signal proportional to the high-side switch current and the COMP pin voltage

are compared each cycle. When the peak switch current intersects the COMP control voltage, the high-side

switch turns off. During overcurrent conditions that pull the output voltage low, the error amplifier increases

switch current by driving the COMP pin high. The error amplifier output clamps internally at a level which sets the

peak switch current limit. The TPS54341 device provides an accurate current limit threshold with a typical current

limit delay of 60 ns. With smaller inductor values, the delay results in a higher peak inductor current. The

relationship between the inductor value and the peak inductor current is shown in Figure 37.

Figure 37. Current Limit Delay

To protect the converter in overload conditions at higher switching frequencies and input voltages, the TPS54341

device implements a frequency foldback. The oscillator frequency is divided by 1, 2, 4, and 8 as the FB pin

voltage falls from 0.8 V to 0 V. The TPS54341 device uses a digital frequency foldback to enable synchronization

to an external clock during normal startup and fault conditions. During short-circuit events, the inductor current

can exceed the peak current-limit because of the high input voltage and the minimum controllable on time. When

the output voltage is forced low by the shorted load, the inductor current decreases slowly during the switch off

time. The frequency foldback effectively increases the off time by increasing the period of the switching cycle

providing more time for the inductor current to ramp down.

With a maximum frequency-foldback ratio of 8, there is a maximum frequency at which frequency-foldback

protection controls the inductor current. Equation 13 calculates the maximum switching frequency at which the

not exceed the calculated value.

Equation 12 calculates the maximum switching-frequency limitation set by the minimum controllable on time and

the input-to-output step-down ratio. Setting the switching frequency above this value causes the regulator to skip

switching pulses to achieve the low duty cycle required to regulate the output voltage at maximum input voltage.

Copyright © 2013, Texas Instruments Incorporated

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