TPS53511

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SLUSBG4 – MARCH 2013

This unique circuit prevents current from being pulled from the output during startup if the output is pre-biased.

When the soft-start commands a voltage higher than the pre-bias level (internal soft-start voltage becomes

greater than feedback voltage VFB), the controller slowly activates synchronous rectification by starting the first

low-side FET gate driver pulses with a narrow on-time. It then increments the on-time on a cycle-by-cycle basis

until it coincides with the time dictated by (1–D), where D is the duty cycle of the converter. This scheme

prevents the initial sinking of the pre-bias output, and ensures that the output voltage (the VO pin) starts and

ramps up smoothly into regulation and the control loop is given time to transition from pre-biased start-up to

normal mode operation.

Power Good

The power good function is activated after soft-start has finished. The power good function becomes active after

1.7 times soft-start time. When the feedback voltage is within ±10% of the target value, internal comparators

detect power good state and the power good signal becomes high. The power good output, PG, is an open drain

output. When the feedback voltage goes ±15% outside of the target value, the power good signal becomes low

after 10-µs internal delay. During an undervoltage condition, when the feedback voltage returns to be within

±10% of the target value, the power good signal goes HIGH again.

Output Discharge Control

TPS53511 discharges the output when EN is low, or the controller is turned off by the protection function (OVP,

UVP, UVLO and thermal shutdown). The output is discharged by an internal 50-

Ω MOSFET which is connected

from VO to PGND. The internal low-side MOSFET is not turned on during the output discharge operation to

avoid the possibility of causing negative voltage at the output.

Current Protection

Output current is limited by cycle-by-cycle overcurrent limiting control. The inductor current is monitored during

the OFF state and the controller keeps the OFF state when the inductor current is larger than the over current

trip level. To provide accuracy and a cost-effective solution, the device supports temperature compensated

The inductor current is monitored by the voltage between the PGND pin and the SW pin. In an overcurrent

condition, the current to the load exceeds the current to the output capacitor; thus, the output voltage tends to fall

off. Eventually the output voltage becomes less than the undervoltage protection threshold and the device shuts

down.

Overvoltage/Undervoltage Protection

The TPS53511 detects over and under-voltage conditions by monitoring the feedback voltage (the VFB pin). This

function is enabled after approximately 1.7 times the soft-start time. When the feedback voltage becomes higher

than 115% of the target voltage, the OVP comparator output goes high and the circuit latches the high-side

MOSFET driver turns off and the low-side MOSFET turns on. Normal operation can be restored only by cycling

the VCC or EN pin voltage. When the feedback voltage becomes lower than 70% of the target voltage, the UVP

comparator output goes high and an internal UVP delay counter begins. After 250 µs, the device latches off both

internal high-side and low-side MOSFET. Similar to the overvoltage protection, the device is latched off, and

normal operation can be restored only by cycling the VCC or EN pin voltage.

UVLO Protection

than UVLO threshold voltage, the TPS53511 is shut off. This protection is non-latching.

Thermal Shutdown

Thermal protection is self-activating. If the junction temperature exceeds the threshold value (typically 150°C),

the TPS53511 shuts off. This protection is non-latching.

Copyright © 2013, Texas Instruments Incorporated

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