TLV705 datasheet(13/33 Pages) TI1

13 / 33 page

TLV705, TLV705P

www.ti.com

SBVS151E – DECEMBER 2010 – REVISED MAY 2015

7.3 Feature Description

7.3.1 Internal Current Limit

The internal current limits of the TLV705 series help protect the regulator during fault conditions. During current

limit, the output sources a fixed amount of current that is largely independent of output voltage. In such a case,

the output voltage is not regulated, and can be measured as VOUT = ILIMIT × RLOAD. The PMOS pass transistor

dissipates [(VIN – VOUT) × ILIMIT] until a thermal shutdown is triggered and the device turns off. When the device

cools down, the internal thermal shutdown circuit turns the device back on. If the fault condition continues, the

device cycles between current limit and thermal shutdown; see the Power Dissipation and Junction Temperature

section for more details.

The PMOS pass element in the TLV705 has a built-in body diode that conducts current when the voltage at VOUT

exceeds the voltage at VIN. This current is not limited, so if extended reverse voltage operation is anticipated,

external limiting to 5% of the rated output current is recommended.

7.3.2 Undervoltage Lockout (UVLO)

The TLV705 uses an UVLO circuit to keep the output shut off until the internal circuitry is operating properly.

7.3.3 Start-Up Current

The TLV705 uses a unique start-up architecture that creates a constant start-up time regardless of the output

capacitor. The start-up current is given by Equation 1. Equation 1 shows that start-up current is directly

proportional to COUT.

ISTARTUP = COUT (μF) × 0.06 (Vμs) + ILOAD (mA)

(1)

The output voltage ramp rate is independent of COUT and the load current, and has a typical value of 0.06 V/μs.

The TLV705 automatically adjusts the soft-start current to supply both the load current and the current to charge

COUT. For example, if ILOAD = 0 mA upon enabling the LDO, then ISTARTUP = 1 μF × 0.06 Vμs + 0 mA = 60 mA,

which is the current that charges the output capacitor.

However, if ILOAD = 200 mA, then ISTARTUP = 1 μF × 0.06 V / μs + 200 mA = 260 mA, which is the current required

for charging the output capacitor and supplying the load current.

If the output capacitor and load are increased such that the start-up current exceeds the output current limit, the

start-up current is clamped at the typical current limit of 400 mA. For example, if COUT = 10 μF and IOUT =

200 mA, then 10

μF × 0.06 V / μs + 200 mA = 800 mA is not supplied and is instead clamped at 400 mA.

7.3.4 Dropout Voltage

The TLV705 uses a PMOS pass transistor to achieve low dropout. When (VIN – VOUT) is less than the dropout

voltage (VDO), the PMOS pass device is in the linear region of operation and the input-to-output resistance is the

RDS(on) of the PMOS pass element. VDO approximately scales with the output current because the PMOS device

functions as a resistor in dropout.

As with any linear regulator, PSRR and transient response are degraded as (VIN – VOUT) approaches dropout.

This effect is shown in Figure 13 in the Typical Characteristics section.

7.3.5 Shutdown

The enable pin (EN) is active high. The device is enabled when the EN pin goes above 0.9 V. This relatively

lower value of voltage required to turn the LDO on can be used to power the device with the GPIO of recent

processors with a GPIO voltage lower than traditional microcontrollers. The device is turned off when the EN pin

is held at less than 0.4 V. When shutdown capability is not required, EN can be connected to the VIN pin. The

TLV705P version has internal active pulldown circuitry that discharges the output with a time constant of:

τ = (120 × RL) / (120 + RL) × COUT

where

•

RL = load resistance

•

COUT = output capacitor

(2)

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