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TPS55340

SLVSBD4B – MAY 2012 – REVISED OCTOBER 2012

www.ti.com

SLOPE COMPENSATION

The TPS55340 has internal slope compensation to prevent sub-harmonic oscillations. The sensed current slope

of boost converter can be expressed as Equation 4:

(4)

The slope compensation dv/dt can be calculated using Equation 5.

(5)

In a converter with current mode control, in addition to the output voltage feedback loop, the inner current loop

including the inductor current sampling effect as well as the slope compensation on the small signal response

should be taken into account, which can be modeled as seen in Equation 6:

(6)

and D is the duty cycle.

Equation 6 is no longer valid.

OVER-CURRENT PROTECTION AND FREQUENCY FOLDBACK

The TPS55340 provides cycle-by-cycle over-current protection that turns off the power switch once the inductor

current reaches the over-current limit threshold. The PWM circuitry resets itself at the beginning of the next

switch cycle. During an over-current event, the output voltage begins to droop as a function of the load on the

output. When the FB voltage through the feedback resistors, drops lower than 0.9 V, the switching frequency is

automatically reduced to 1/4 of the normal value. Figure 7 shows the non-foldback frequency with an 80k

Ω timing

resistor and the corresponding foldback frequency. The switching frequency does not return to normal until the

over-current condition is removed and the FB voltage increases above 0.9 V. The frequency foldback feature is

disabled during soft-start.

ENABLE AND THERMAL SHUTDOWN

The TPS55340 enters shutdown when the EN voltage is less than 0.68 V (min) for more than 1ms. In shutdown,

the input supply current for the device is less than 10µA (max). The EN pin has an internal 950k

Ω pull down

resistor to disable the device if the pin is floating.

An internal thermal shutdown turns off the device when the junction temperature exceeds 165°C (typical). The

device will restart when the junction temperature drops by 15°C.

UNDER-VOLTAGE LOCKOUT (UVLO)

An under-voltage lockout circuit prevents mis-operation of the device at input voltages below 2.5 V (typical).

When the input voltage is below the UVLO threshold, the device remains off and the internal power MOSFET is

turned off. The UVLO threshold is set below minimum operating voltage of 2.9 V to ensure that a transient VIN

dip will not cause the device to reset. For the input voltages between UVLO threshold and 2.9 V, the device

attempts to operate, but the electrical specifications are not guaranteed.

10

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