TPS61080

TPS61081

SLVS644D – FEBRUARY 2006 – REVISED APRIL 2013

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DETAILED DESCRIPTION

OPERATION

TPS61080/1 is a highly integrated boost regulator for up to 27V output. In addition to the on-chip 0.5A/1.2A PWM

switch and power diode, this IC also builds in an input side isolation switch as shown in the block diagram. One

common issue with conventional boost regulator is the conduction path from input to output even when PWM

switch is turned off. It creates three problems, inrush current during start up, output leakage voltage under

shutdown, and unlimited short circuit current. To address these issues, TPS61080/1 turns off the isolation switch

under shutdown mode and short circuit condition to eliminate any possible current path.

TPS61080/1 adopts current mode control with constant PWM (pulse width modulation) frequency. The switching

frequency can be configured to either 600KHz or 1.2MHz through the FSW pin. 600KHz improves light load

efficiency, while 1.2MHz allows using smaller external component. The PWM operation turns on the PWM switch

at the beginning of each switching cycle. The input voltage is applied across the inductor and stores the energy

as inductor current ramps up. The load current is provided by the output capacitor. When the inductor current

across the threshold set by error amplifier output, the PWM switch is turned off, and the power diode is forward

biased. The inductor transfers its stored energy to replenish the output capacitor. This operation repeats in the

next switching cycle.

The error amplifier compares the FB pin voltage with an internal reference, and its output determines the duty

cycle of the PWM switching. This close loop system requires loop compensation for stable operation.

TPS61080/1 has internal compensation circuitry which accommodates a wide range of input and output voltages.

The TPS61080/1 integrates slope compensation to the current ramp to avoid the sub-harmonic oscillation that is

intrinsic to current mode control schemes.

START UP

TPS61080/1 turns on the isolation FET when the EN pin is pulled high, provided that the input voltage is higher

than the undervoltage lockout threshold. The Vgs of the isolation FET is clamped to maintain high on-resistance

and limits the current to 30mA charging the output capacitor. This feature limits the in-rush current and maximum

start up current to 30mA. Once the output capacitor is charged to VIN, the IC removes the Vgs clamp to fully turn

on the isolation FET and at the same time actives soft start by charging the capacitor on the SS pin. If OUT stays

lower than VIN-Vsc following a 1.7ms delay after enable is taken high, the IC recognizes a short circuit condition.

In this case, the isolation FET turns off, and IC remains off until the EN pin toggles or VIN cycles through power

on reset (POR).

During the soft start phase, the SS pin capacitor is charged by internal bias current of the SS pin. The SS pin

capacitor programs the ramp up slope. The SS pin voltage clamps the reference voltage of the FB pin, therefore

the output capacitor rise time follows the SS pin voltage. Without the soft start, the inductor current could reach

the over current limit threshold, and there is potential for output overshoot. see the APPLICATION

INFORMATION section on selecting soft start capacitor values. Pulling the SS pin to ground disables the PWM

switching. However, unlike being disabled by pulling EN low, the IC continues to draw quiescent current and the

isolation FET remains on.

OVERCURRENT AND SHORT CIRCUIT PROTECTION

TPS61080/1 has a pulse by pulse over current limit feature which turns off the power switch once the inductor

current reaches the overcurrent limit. The PWM circuitry resets itself at the beginning of the next switch cycle.

The overcurrent threshold determines the available output current. However, the maximum output is also a

function of the input voltage, output voltage, switching frequency and inductor value. Larger inductor values and

1.2MHz switching frequency increase the current output capability because of the reduced current ripple. See the

APPLICATION INFORMATION section for the maximum output current calculation.

In typical boost converter topologies, if the output is grounded, turning off the power switch does not limit the

current because a current path exists from the input to output through the inductor and power diode. To eliminate

this path, TPS61080/1 turns off the isolation FET between the input and the inductor. This circuit is triggered

when the inductor current remains above short circuit current limit for more than 13

μs, or the OUT pin voltage

falls below VIN-1.4V for more than 1.7ms. An internal catch-diode between the L pin and ground turns on to

provide a current discharge path for the inductor. If the short is caused by the output being low, then the IC shuts

down and waits for EN to be toggled or a POR. If the short protection is triggered by short circuit current limit, the

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