TPS737 datasheet(21/50 Pages) TI2

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7.2.2.1 Input and Output Capacitor Requirements

Although an input capacitor is not required for stability if input impedance is very low, good analog design

practice is to connect a 0.1-μF to 1-μF low equivalent series resistance (ESR) capacitor across the input supply

near the regulator. This capacitor counteracts reactive input sources and improves transient response, noise

rejection, and ripple rejection. A higher-value capacitor can be necessary if large, fast rise-time load transients

are anticipated or the device is located several inches from the power source.

The TPS737 requires a 1-μF output capacitor for stability. The device is designed to be stable for all available

types and values of capacitors. In applications where multiple low-ESR capacitors are in parallel, ringing

can occur when the product of COUT and total ESR drops below 50 nF·Ω. Total ESR includes all parasitic

resistances, including capacitor ESR and board, socket, and solder joint resistance. In most applications, the

sum of capacitor ESR and trace resistance meets this requirement.

7.2.2.2 Dropout Voltage

The TPS737 uses an NMOS pass transistor to achieve extremely low dropout. When (VIN – VOUT) is less than

the dropout voltage (VDO), the NMOS pass transistor is in the linear region of operation and the input-to-output

resistance is the RDS(on) of the NMOS pass transistor.

For large step changes in load current, the TPS737 requires a larger voltage drop from VIN to VOUT to avoid

degraded transient response. The boundary of this transient dropout region is approximately twice the DC

dropout. Values of (VIN – VOUT) above this line provide normal transient response.

Operating in the transient dropout region can cause an increase in recovery time. The time required to recover

from a load transient is a function of the magnitude of the change in load current rate, the rate of change in

load current, and the available headroom (VIN-to-VOUT voltage drop). Under worst-case conditions [full-scale

instantaneous load change with (VIN – VOUT) close to DC dropout levels], the TPS737 can take a couple of

hundred microseconds to return to the specified regulation accuracy.

7.2.2.3 Transient Response

The low open-loop output impedance provided by the NMOS pass transistor in a voltage-follower configuration

allows operation without a 1-µF output capacitor. As with any regulator, the addition of additional capacitance

from the OUT pin to ground reduces undershoot magnitude but increases undershoot duration. In the adjustable

version, the addition of a capacitor, CFB, from the OUT pin to the FB pin also improves the transient response.

The TPS737 does not have an active pulldown when the output is overvoltage. This architecture allows

applications that connect higher voltage sources, such as alternate power supplies, to the output. This

architecture also results in an output overshoot of several percent if the load current quickly drops to zero when

a capacitor is connected to the output. The duration of overshoot can be reduced by adding a load resistor. The

overshoot decays at a rate determined by output capacitor COUT and the internal and external load resistance.

The rate of decay is given by:

(Fixed voltage version)

OUT

LOAD

80k

W P

(4)

(Adjustable voltage version)

OUT

LOAD

80k

R ) R

(5)

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TPS737

SBVS067W – JANUARY 2006 – REVISED AUGUST 2025

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