TPS780 datasheet(17/27 Pages) TI

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INTERNAL CURRENT LIMIT

TPS780

GND

SET

OUT

1 F

4.2Vto5.5V

2.2V

DROPOUT VOLTAGE

SHUTDOWN

t=3

10k

10kW +RL

OUT

(3)

TRANSIENT RESPONSE

ACTIVE VOUT PULL-DOWN

MINIMUM LOAD

TPS780

GND

SET

OUT

High=V

SET

OUT(LOW)

Low=V

SET

OUT(HIGH)

OUT

1 F

4.2Vto5.5V

2.2Vto3.3V

TPS780 Series

www.ti.com ....................................................................................................................................................... SBVS083C – JANUARY 2007 – REVISED MAY 2008

The TPS780 series are internally current-limited to

protect the regulator during fault conditions. During

current limit, the output sources a fixed amount of

current that is largely independent of output voltage.

For reliable operation, the device should not be

operated in a current limit state for extended periods

of time.

The PMOS pass element in the TPS780 series has a

built-in body diode that conducts current when the

voltage at OUT exceeds the voltage at IN. This

current is not limited, so if extended reverse voltage

operation is anticipated, external limiting to 5% of

Figure 59. Circuit to Tie Both EN and VSET High

rated output current may be appropriate.

The TPS780 series use a PMOS pass transistor to

The enable pin (EN) is active high and is compatible

achieve low dropout. When (VIN – VOUT) is less than

with standard and low-voltage CMOS levels. When

the dropout voltage (VDO), the PMOS pass device is

shutdown capability is not required, EN should be

the linear region of operation and the input-to-output

connected to the IN pin, as shown in Figure 58.

resistance is the RDS(ON) of the PMOS pass element.

Figure 59 shows both EN and VSET connected to IN.

VDO

approximately

scales

with

output

current

The TPS780 series, with internal active output

because the PMOS device behaves like a resistor in

pull-down circuitry, discharges the output to within 5%

dropout. As with any linear regulator, PSRR and

VOUT with a time (t) shown in Equation 3:

transient response are degraded as (VIN – VOUT)

approaches dropout. This effect is shown in the

Typical Characteristics section. Refer to application

report

SLVA207,

Understanding

LDO

Dropout,

available for download from www.ti.com.

Where:

RL= output load resistance

COUT = output capacitance

As with any regulator, increasing the size of the

output capacitor reduces over/undershoot magnitude

but increases duration of the transient response. For

more information, see Figure 48.

In the TPS780 series, the active pull-down discharges

VOUT when the device is off. However, the input

voltage must be greater than 2.2V for the active

pull-down to work.

The TPS780 series are stable with no output load.

Figure 58. Circuit Showing EN Tied High when

Traditional PMOS LDO regulators suffer from lower

Shutdown Capability is Not Required

loop gain at very light output loads. The TPS780

series employ an innovative, low-current circuit under

very light or no-load conditions, resulting in improved

output voltage regulation performance down to zero

output current. See Figure 47 for the load transient

response.

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