TPS73601DBVR datasheet(13/32 Pages) TI

13 / 32 page

N(mVRMS) + 8.5

mVRMS

OUT(V)

TPS736xx

www.ti.com

SBVS038T – SEPTEMBER 2003 – REVISED AUGUST 2010

An internal 27k

Ω resistor in series with the noise

ENABLE PIN AND SHUTDOWN

reduction pin (NR) forms a low-pass filter for the

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

voltage reference when an external noise reduction

with standard TTL-CMOS levels. A VEN below 0.5V

capacitor, CNR, is connected from NR to ground. For

(max) turns the regulator off and drops the GND pin

CNR = 10nF, the total noise in the 10Hz to 100kHz

current to approximately 10nA. When EN is used to

bandwidth is reduced by a factor of ~3.2, giving the

shutdown the regulator, all charge is removed from

approximate relationship:

the pass transistor gate, and the output ramps back

up to a regulated VOUT (see Figure 23).

(3)

When shutdown capability is not required, EN can be

connected to VIN. However, the pass gate may not be

for CNR = 10nF.

discharged using this configuration, and the pass

This noise reduction effect is shown as RMS Noise

transistor may be left on (enhanced) for a significant

Voltage vs CNR in the Typical Characteristics section.

time after VIN has been removed. This scenario can

result in reverse current flow (if the IN pin is low

The TPS73601 adjustable version does not have the

impedance) and faster ramp times upon power-up. In

NR pin available. However, connecting a feedback

addition, for VIN ramp times slower than a few

capacitor, CFB, from the output to the feedback pin

milliseconds,

the

output

may

overshoot

upon

(FB)

reduces

output

noise

and

improves

load

power-up.

transient performance.

Note that current limit foldback can prevent device

The TPS736xx uses an internal charge pump to

start-up under some conditions. See the Internal

develop an internal supply voltage sufficient to drive

Current Limit section for more information.

the gate of the NMOS pass element above VOUT. The

charge pump generates ~250mV of switching noise at

DROPOUT VOLTAGE

~4MHz; however, charge-pump noise contribution is

negligible at the output of the regulator for most

The TPS736xx uses an NMOS pass transistor to

values of IOUT and COUT.

achieve extremely low dropout. When (VIN – VOUT) is

less than the dropout voltage (VDO), the NMOS pass

BOARD LAYOUT RECOMMENDATION TO

device is in its linear region of operation and the

IMPROVE PSRR AND NOISE PERFORMANCE

input-to-output resistance is the RDS-ON of the NMOS

pass element.

To improve ac performance such as PSRR, output

noise, and transient response, it is recommended that

For large step changes in load current, the TPS736xx

the board be designed with separate ground planes

requires a larger voltage drop from VIN to VOUT to

for VIN and VOUT, with each ground plane connected

avoid degraded transient response. The boundary of

only at the GND pin of the device. In addition, the

this transient dropout region is approximately twice

ground connection for the bypass capacitor should

the dc dropout. Values of VIN – VOUT above this line

connect directly to the GND pin of the device.

ensure normal transient response.

Operating in the transient dropout region can cause

INTERNAL CURRENT LIMIT

an increase in recovery time. The time required to

recover from a load transient is a function of the

The TPS736xx internal current limit helps protect the

magnitude of the change in load current rate, the rate

regulator during fault conditions. Foldback current

change

load

current,

and

the

available

limit helps to protect the regulator from damage

headroom

(VIN to VOUT voltage drop). Under

during output short-circuit conditions by reducing

worst-case conditions [full-scale instantaneous load

current limit when VOUT drops below 0.5V. See

change with (VIN – VOUT) close to dc dropout levels],

Figure 12 in the Typical Characteristics section.

the TPS736xx can take a couple of hundred

Note from Figure 12 that approximately –0.2V of VOUT

microseconds to return to the specified regulation

results in a current limit of 0mA. Therefore, if OUT is

accuracy.

forced below –0.2V before EN goes high, the device

may not start up. In applications that work with both a

positive and negative voltage supply, the TPS736xx

should be enabled first.

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