-

=

Vo

Vref

R5

R6

Vref

12

TPS54821

SLVSB14B – OCTOBER 2011 – REVISED FEBRUARY 2016

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Copyright © 2011–2016, Texas Instruments Incorporated

Feature Description (continued)

If tied together, the input voltage for VIN and PVIN can range from 4.5 V to 17 V. If using the VIN separately from

PVIN, the VIN pin must be between 4.5 V and 17 V, and the PVIN pin can range from as low as 1.6 V to 17 V. A

voltage divider connected to the EN pin can adjust the either input voltage UVLO appropriately. Adjusting the

input voltage UVLO on the PVIN pin helps to provide consistent power up behavior.

7.3.4 Voltage Reference

The voltage reference system produces a precise ±1% voltage reference over temperature by scaling the output

of a temperature stable bandgap circuit.

7.3.5 Adjusting the Output Voltage

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

use 1% tolerance or better divider resistors. Referring to the application schematic of Figure 29, start with a 10

k

Ω for R6 and use Equation 1 to calculate R5. To improve efficiency at 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.

(1)

Where Vref = 0.6V

The minimum output voltage and maximum output voltage can be limited by the minimum on time of the high-

side MOSFET and bootstrap voltage (BOOT-PH voltage) respectively. More discussions are located in Minimum

Output Voltage and Bootstrap Voltage (BOOT) and Low Dropout Operation.

7.3.6 Safe Start-up into Pre-Biased Outputs

The device has been designed to prevent the low-side MOSFET from discharging a pre-biased output. During

monotonic pre-biased startup, the low-side MOSFET is not allowed to sink current until the SS/TR pin voltage is

higher than 1.4 V.

7.3.7 Error Amplifier

The device uses a transconductance error amplifier. The error amplifier compares the VSENSE pin voltage to the

lower of the SS/TR pin voltage or the internal 0.6 V voltage reference. The transconductance of the error

amplifier is 1300

μA/V during normal operation. The frequency compensation network is connected between the

COMP pin and ground.

7.3.8 Slope Compensation

The device adds a compensating ramp to the switch current signal. This slope compensation prevents sub-

harmonic oscillations. The available peak inductor current remains constant over the full duty cycle range.

7.4 Device Functional Modes

7.4.1 Enable and Adjusting Undervoltage Lockout

The EN pin provides electrical on/off control of the device. Once the EN pin voltage exceeds the threshold

voltage, the device starts operation. If the EN pin voltage is pulled below the threshold voltage, the regulator

stops switching and enters low Iq state.

The EN pin has an internal pull-up current source, allowing the user to float the EN pin for enabling the device. If

an application requires controlling the EN pin, use open drain or open collector output logic to interface with the

pin.

The device implements internal UVLO circuitry on the VIN pin. The device is disabled when the VIN pin voltage

falls below the internal VIN UVLO threshold. The internal VIN UVLO threshold has a hysteresis of 150mV.

If an application requires either a higher UVLO threshold on the VIN pin or a secondary UVLO on the PVIN, in

split rail applications, then the EN pin can be configured as shown in Figure 17, Figure 18 and Figure 19. When

using the external UVLO function it is recommended to set the hysteresis to be greater than 500mV.