Feed Forward Gain +

VIN

Ramp

pk*pk

TPS5430

TPS5431

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SLVS632E – JANUARY 2006 – REVISED SEPTEMBER 2013

Undervoltage Lockout (UVLO)

The TPS5430/TPS5431 incorporates an undervoltage lockout circuit to keep the device disabled when VIN (the

input voltage) is below the UVLO start voltage threshold. During power up, internal circuits are held inactive and

the internal slow start is grouded until VIN exceeds the UVLO start threshold voltage. Once the UVLO start

threshold voltage is reached, the internal slow start is released and device start-up begins. The device operates

until VIN falls below the UVLO stop threshold voltage. The typical hysteresis in the UVLO comparator is 330 mV.

Boost Capacitor (BOOT)

Connect a 0.01

μF low-ESR ceramic capacitor between the BOOT pin and PH pin. This capacitor provides the

gate drive voltage for the high-side MOSFET. X7R or X5R grade dielectrics are recommended due to their stable

values over temperature.

Output Feedback (VSENSE) and Internal Compensation

The output voltage of the regulator is set by feeding back the center point voltage of an external resistor divider

network to the VSENSE pin. In steady-state operation, the VSENSE pin voltage should be equal to the voltage

reference 1.221 V.

The TPS5430/TPS5431 implements internal compensation to simplify the regulator design. Since the

TPS5430/TPS5431 uses voltage mode control, a type 3 compensation network has been designed on chip to

provide a high crossover frequency and a high phase margin for good stability. See the Internal Compensation

Network in the applications section for more details.

Voltage Feed Forward

The internal voltage feed forward provides a constant dc power stage gain despite any variations with the input

voltage. This greatly simplifies the stability analysis and improves the transient response. Voltage feed forward

varies the peak ramp voltage inversely with the input voltage so that the modulator and power stage gain are

constant at the feed forward gain, i.e.

(1)

The typical feed forward gain of TPS5430/TPS5431 is 25.

Pulse-Width-Modulation (PWM) Control

The regulator employs a fixed frequency pulse-width-modulator (PWM) control method. First, the feedback

voltage (VSENSE pin voltage) is compared to the constant voltage reference by the high gain error amplifier and

compensation network to produce a error voltage. Then, the error voltage is compared to the ramp voltage by the

PWM comparator. In this way, the error voltage magnitude is converted to a pulse width which is the duty cycle.

Finally, the PWM output is fed into the gate drive circuit to control the on-time of the high-side MOSFET.

Overcurrent Limiting

Overcurrent limiting is implemented by sensing the drain-to-source voltage across the high-side MOSFET. The

drain to source voltage is then compared to a voltage level representing the overcurrent threshold limit. If the

drain-to-source voltage exceeds the overcurrent threshold limit, the overcurrent indicator is set true. The system

will ignore the overcurrent indicator for the leading edge blanking time at the beginning of each cycle to avoid any

turn-on noise glitches.

Once overcurrent indicator is set true, overcurrent limiting is triggered. The high-side MOSFET is turned off for

the rest of the cycle after a propagation delay. The overcurrent limiting mode is called cycle-by-cycle current

limiting.

Sometimes under serious overload conditions such as short-circuit, the overcurrent runaway may still happen

when using cycle-by-cycle current limiting. A second mode of current limiting is used, i.e. hiccup mode

overcurrent limiting. During hiccup mode overcurrent limiting, the voltage reference is grounded and the high-side

MOSFET is turned off for the hiccup time. Once the hiccup time duration is complete, the regulator restarts under

control of the slow start circuit.

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