TPS73701 datasheet(11/18 Pages) TI

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BOARD LAYOUT RECOMMENDATION TO

TRANSIENT RESPONSE

INTERNAL CURRENT LIMIT

SHUTDOWN

OUT

80kW ø RLOAD

(3)

DROPOUT VOLTAGE

OUT

80kW ø (R1 ) R2) ø RLOAD

(4)

REVERSE CURRENT

TPS737xx

SBVS067C – JANUARY 2006 – REVISED AUGUST 2006

IMPROVE PSRR AND NOISE

The low open-loop output impedance provided by the

PERFORMANCE

NMOS

pass

element

voltage

follower

To improve ac performance such as PSRR, output

configuration allows operation without a 1.0µF output

noise, and transient response, it is recommended

capacitor. As with any regulator, the addition of

that the printed circuit board (PCB) be designed with

additional capacitance from the output pin to ground

separate ground planes for VIN and VOUT, with each

reduces

undershoot

magnitude

but

increases

ground plane connected only at the GND pin of the

duration. In the adjustable version, the addition of a

device. In addition, the ground connection for the

capacitor, CFB, from the output to the adjust pin will

bypass capacitor should connect directly to the GND

also improve the transient response.

pin of the device.

The TPS737xx does not have active pull-down when

the output is over-voltage. This architecture allows

applications that connect higher voltage sources,

such as alternate power supplies, to the output. This

The TPS737xx internal current limit helps protect the

architecture also results in an output overshoot of

regulator during fault conditions. Foldback helps to

several percent if the load current quickly drops to

protect the regulator from damage during output

zero when a capacitor is connected to the output.

short-circuit conditions by reducing current limit when

The duration of overshoot can be reduced by adding

VOUT drops below 0.5V.

a load resistor. The overshoot decays at a rate

determined

output

capacitor

COUT and the

internal/external load resistance. The rate of decay is

The Enable pin is active high and is compatible with

given by:

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

(Fixed voltage version)

turns the regulator off and drops the ground pin

current to approximately 10nA. When shutdown

capability is not required, the Enable pin can be

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

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

(Adjustable voltage version)

resistor values below 50k

Ω.

The TPS737xx uses an NMOS pass transistor to

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

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

The NMOS pass element of the TPS737xx provides

device is in its linear region of operation and the

inherent protection against current flow from the

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

output of the regulator to the input when the gate of

pass element.

the pass device is pulled low. To ensure that all

charge is removed from the gate of the pass

For

large

step

changes

load

current,

the

element, the enable pin must be driven low before

TPS737xx requires a larger voltage drop from VIN to

the input voltage is removed. If this is not done, the

VOUT to avoid degraded transient response. The

pass element may be left on because of stored

boundary

this

transient

dropout

region

charge on the gate.

approximately twice the dc dropout. Values of VIN –

VOUT above this line ensure normal transient

After the enable pin is driven low, no bias voltage is

response.

needed on any pin for reverse current blocking. Note

that reverse current is specified as the current

Operating in the transient dropout region can cause

flowing out of the IN pin because of voltage applied

an increase in recovery time. The time required to

on the OUT pin. There will be additional current

recover from a load transient is a function of the

flowing into the OUT pin as a result of the 80k

Ω

magnitude of the change in load current rate, the

internal resistor divider to ground (see Figure 1 and

rate of change in load current, and the available

Figure 2).

headroom

(VIN to VOUT voltage drop). Under

worst-case conditions [full-scale instantaneous load

For the TPS73701, reverse current may flow when

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

VFB is more than 1.0V above VIN.

the TPS737xx can take a couple of hundred

microseconds to return to the specified regulation

accuracy.

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