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TPS54478

www.ti.com

SLVSAS2

– JUNE 2011

To reduce the power dissipation of TPS54478 during overcurrent event, the hiccup protection is implemented

beyond the cycle-by-cycle protection. Thermal shutdown prevents the overheat damage of the device.

DETAILED DESCRIPTION

FIXED FREQUENCY PWM CONTROL

The TPS54478 uses an adjustable fixed frequency, peak current mode control. The output voltage is compared

through external resistors on the VSENSE pin to an internal voltage reference by an error amplifier which drives

the COMP pin. An internal oscillator initiates the turn on of the high side power switch. The error amplifier output

is compared to the high side power switch current. When the power switch current reaches the COMP voltage

level the high side power switch is turned off and the low side power switch is turned on. The COMP pin voltage

increases and decreases as the output current increases and decreases. The device implements a current limit

by clamping the COMP pin voltage to a maximum level and also implements a minimum clamp for improved

transient response performance.

SLOPE COMPENSATION AND OUTPUT CURRENT

The TPS54478 adds a compensating ramp to the switch current signal. This slope compensation prevents

sub-harmonic oscillations as duty cycle increases. The available peak inductor current remains constant over the

full duty cycle range.

BOOTSTRAP VOLTAGE (BOOT) AND LOW DROPOUT OPERATION

The TPS54478 has an integrated boot regulator and requires a small ceramic capacitor between the BOOT and

PH pin to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor should be

0.1

μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is

recommended because of the stable characteristics over temperature and voltage.

To improve drop out, the TPS54478 is designed to operate at 100% duty cycle as long as the BOOT to PH pin

voltage is greater than 2.2 V. The high side MOSFET is turned off using an UVLO circuit, allowing for the low

side MOSFET to conduct when the voltage from BOOT to PH drops below 2.2 V. Since the supply current

sourced from the BOOT pin is very low, the high side MOSFET can remain on for more switching cycles than are

required to refresh the capacitor, thus the effective duty cycle of the switching regulator is very high.

ERROR AMPLIFIER

The TPS54478 has a transconductance amplifier. The error amplifier compares the VSENSE voltage to the lower

of the SS/TR pin voltage or the internal 0.600 V voltage reference. The transconductance of the error amplifier is

225

μA/V during normal operation. When the voltage of VSENSE pin is below 0.600 V and the device is

regulating using the SS/TR voltage, the gm is typically greater than 77

μA/V, but less than 225 μA/V. The

frequency compensation components are placed between the COMP pin and ground.

VOLTAGE REFERENCE

The voltage reference system produces a precise

±1% voltage reference over temperature by scaling the output

of a temperature-stable bandgap circuit. The bandgap and scaling circuits produce 0.600 V at the non-inverting

input of the error amplifier.

ADJUSTING THE OUTPUT VOLTAGE

The output voltage is set with a resistor divider from the output node to the VSENSE pin. It is recommended to

use divider resistors with 1% tolerance or better. Start with a 20 k

Ω for the R1 resistor and use the Equation 1 to

calculate R2. To improve efficiency at very light loads consider using larger value resistors. If the values are too

high the regulator is more susceptible to noise and voltage errors from the VSENSE input current are noticeable.

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Copyright

© 2011, Texas Instruments Incorporated

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