TPS73601DBVT datasheet(12/26 Pages) TI

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Shutdown

N(mVRMS)

+ 8.5

mVRMS

OUT(V)

(3)

Dropout Voltage

Board Layout Recommendation to Improve

Internal Current Limit

Transient Response

TPS736xx

SBVS038Q – SEPTEMBER 2003 – REVISED AUGUST 2007

An internal 27k

Ω resistor in series with the noise

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

The Enable pin is active high and is compatible with

voltage reference when an external noise reduction

standard TTL-CMOS levels. VEN below 0.5V (max)

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

turns the regulator off and drops the ground pin

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

current to approximately 10nA. When shutdown

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

capability is not required, the Enable pin can be

approximate relationship:

connected to VIN. When a pull-up resistor is used,

and operation down to 1.8V is required, use pull-up

resistor values below 50k

Ω. To ensure all charge is

removed from the gate of the pass element, the

for CNR = 10nF.

Enable pin must be driven low before the input

voltage is removed. If the Enable pin is not driven

This noise reduction effect is shown as RMS Noise

low, the pass element may be left on because of

Voltage vs CNR in the Typical Characteristics section.

stored charge on the gate.

The TPS73601 adjustable version does not have the

noise-reduction pin available. However, connecting a

feedback capacitor, CFB, from the output to the FB pin

The TPS736xx uses an NMOS pass transistor to

reduces output noise and improves load transient

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

performance.

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

The TPS736xx uses an internal charge pump to

device is in its linear region of operation and the

develop an internal supply voltage sufficient to drive

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

the gate of the NMOS pass element above VOUT. The

pass element.

charge pump generates ~250

μV of switching noise at

For large step changes in load current, the TPS736xx

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

requires a larger voltage drop from VIN to VOUT to

negligible at the output of the regulator for most

avoid degraded transient response. The boundary of

values of IOUT and COUT.

this transient dropout region is approximately twice

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

ensure normal transient response.

PSRR and Noise Performance

Operating in the transient dropout region can cause

To improve ac performance such as PSRR, output

an increase in recovery time. The time required to

noise, and transient response, it is recommended that

recover from a load transient is a function of the

the board be designed with separate ground planes

magnitude of the change in load current rate, the rate

for VIN and VOUT, with each ground plane connected

change

load

current,

and

the

available

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

headroom

(VIN to VOUT voltage drop). Under

ground connection for the bypass capacitor should

worst-case conditions [full-scale instantaneous load

connect directly to the GND pin of the device.

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

the TPS736xx can

take

couple

hundred

microseconds to return to the specified regulation

accuracy.

The TPS736xx internal current limit helps protect the

regulator during fault conditions. Foldback current

limit helps to protect the regulator from damage

during output short-circuit conditions by reducing

The low open-loop output impedance provided by the

current limit when VOUT drops below 0.5V. See

NMOS

pass

element

voltage

follower

Figure 12 in the Typical Characteristics section.

configuration allows operation without an output

capacitor

for

many

applications.

with

any

regulator, the addition of a capacitor (nominal value

μF) from the output pin to ground will reduce

undershoot magnitude but increase its duration. In

the adjustable version, the addition of a capacitor,

CFB, from the output to the adjust pin will also

improve the transient response.

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