TPS73233DBVT datasheet(12/23 Pages) TI

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DROPOUT VOLTAGE

N(mVRMS)

+ 8.5

mVRMS

OUT(V)

(3)

BOARD LAYOUT RECOMMENDATION TO

TRANSIENT RESPONSE

INTERNAL CURRENT LIMIT

SHUTDOWN

dV dt

OUT

80kW ø RLOAD

(4)

TPS732xx

SBVS037I – AUGUST 2003 – REVISED MAY 2006

The TPS732xx uses an NMOS pass transistor to

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

for CNR = 10nF.

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

device is in its linear region of operation and the

This noise reduction effect is shown as RMS Noise

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

Voltage vs CNR in the Typical Characteristics section.

pass element.

The TPS73201 adjustable version does not have the

For

large

step

changes

load

current,

the

noise-reduction pin available. However, connecting a

TPS732xx requires a larger voltage drop from VIN to

feedback capacitor, CFB, from the output to the FB

VOUT to avoid degraded transient response. The

pin will reduce output noise and improve load

boundary

this

transient

dropout

region

transient performance.

approximately twice the dc dropout. Values of VIN -

The TPS732xx uses an internal charge pump to

VOUT above this line insure normal transient

develop an internal supply voltage sufficient to drive

response.

the gate of the NMOS pass element above VOUT.

Operating in the transient dropout region can cause

The charge pump generates ~250µV of switching

an increase in recovery time. The time required to

noise

~2MHz;

however,

charge-pump

noise

recover from a load transient is a function of the

contribution is negligible at the output of the regulator

magnitude of the change in load current rate, the

for most values of IOUT and COUT.

rate of change in load current, and the available

headroom

(VIN to VOUT voltage drop). Under

worst-case conditions [full-scale instantaneous load

IMPROVE PSRR AND NOISE

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

PERFORMANCE

the TPS732xx can take a couple of hundred

To improve ac performance such as PSRR, output

microseconds to return to the specified regulation

noise, and transient response, it is recommended

accuracy.

that the PCB be designed with separate ground

planes for VIN and VOUT, with each ground plane

connected only at the GND pin of the device. In

The low open-loop output impedance provided by the

addition, the ground connection for the bypass

NMOS

pass

element

voltage

follower

capacitor should connect directly to the GND pin of

configuration allows operation without an output

the device.

capacitor

for

many

applications.

with

any

regulator, the addition of a capacitor (nominal value

1µF) from the output pin to ground will reduce

The TPS732xx internal current limit helps protect the

undershoot magnitude but increase duration. In the

regulator during fault conditions. Foldback helps to

adjustable version, the addition of a capacitor, CFB,

protect the regulator from damage during output

from the output to the adjust pin will also improve the

short-circuit conditions by reducing current limit when

transient response.

VOUT drops below 0.5V. See Figure 11 in the Typical

The TPS732xx does not have active pull-down when

Characteristics section for a graph of IOUT vs VOUT.

the output is over-voltage. This allows applications

that

connect

higher

voltage

sources,

such

alternate power supplies, to the output. This also

results in an output overshoot of several percent if

The Enable pin is active high and is compatible with

the load current quickly drops to zero when a

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

capacitor is connected to the output. The duration of

turns the regulator off and drops the ground pin

overshoot can be reduced by adding a load resistor.

current to approximately 10nA. When shutdown

The overshoot decays at a rate determined by output

capability is not required, the Enable pin can be

capacitor

COUT and the

internal/external

load

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

resistance. The rate of decay is given by:

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

resistor values below 50 k

Ω.

(Fixed voltage version)

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