LT1764AET-2.5 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1764AET-2.5

3A, Fast Transient Response, Low Noise, LDO Regulators

Linear Technology 

LT1764A Series

APPLICATIO S I FOR ATIO

ing the capacitor value improves the stability and reduces

the ringing. Table 1 gives some recommended values of

ESR to minimize ringing caused by fast, hard current

transitions.

Table 1. Capacitor Minimum ESR

VOUT

10µF

22µF

1.2V

10mΩ

5mΩ

1.5V

7mΩ

5mΩ

1.8V

5mΩ

5mΩ

2.5V

0mΩ

0mΩ

3.3V

0mΩ

0mΩ

≥ 5V

0mΩ

0mΩ

47µF

3mΩ

3mΩ

3mΩ

0mΩ

0mΩ

0mΩ

100µF

0mΩ

0mΩ

0mΩ

0mΩ

0mΩ

0mΩ

Figures 3 through 8 show the effect of ESR on the transient

response of the regulator. These scope photos show the

transient response for the LT1764A at three different

output voltages with various capacitors and various val-

ues of ESR. The output load conditions are the same for all

traces. In all cases there is a DC load of 1A. The load steps

up to 2A at the first transition and steps back to 1A at the

second transition.

At the worst case point of 1.2VOUT with 10µF COUT

(Figure 3), a minimum amount of ESR is required. While

5mΩ is enough to eliminate most of the ringing, a value

closer to 20mΩ provides a more optimum response. At

2.5V output with 10µF COUT (Figure 4) the output rings

at the transitions with 0Ω ESR but still settles to within

10mV in 20µs after the 1A load step. Once again a small

value of ESR will provide a more optimum response.

At 5VOUT with 10µF COUT (Figure 5) the response is well

damped with 0Ω ESR.

With a COUT of 100µF at 0Ω ESR and an output of 1.2V

(Figure 6), the output rings although the amplitude is only

10mVp-p. With COUT of 100µF it takes only 5mΩ to 20mΩ

of ESR to provide good damping at 1.2V output. Perfor-

mance at 2.5V and 5V output with 100µF COUT shows sim-

ilar characteristics to the 10µF case (see Figures 7-8). At

2.5VOUT 5mΩ to 20mΩ can improve transient response.

At 5VOUT the response is well damped with 0Ω ESR.

Capacitor types with inherently higher ESR can be com-

bined with 0mΩ ESR ceramic capacitors to achieve both

good high frequency bypassing and fast settling time.

Figure 9 illustrates the improvement in transient response

that can be seen when a parallel combination of ceramic

and POSCAP capacitors are used. The output voltage is at

the worst case value of 1.2V. Trace A, is with a 10µF

ceramic output capacitor and shows significant ringing

with a peak amplitude of 25mV. For Trace B, a 22µF/45mΩ

POSCAP is added in parallel with the 10µF ceramic. The

output is well damped and settles to within 10mV in less

than 5µs.

For Trace C, a 100µF/35mΩ POSCAP is connected in

parallel with the 10µF ceramic capacitor. In this case the

peak output deviation is less than 20mV and the output

settles in about 5µs. For improved transient response the

value of the bulk capacitor (tantalum or aluminum electro-

lytic) should be greater than twice the value of the ceramic

capacitor.

Tantalum and Polytantalum Capacitors

There is a variety of tantalum capacitor types available,

with a wide range of ESR specifications. Older types have

ESR specifications in the hundreds of mΩ to several

Ohms. Some newer types of polytantalum with multi-

electrodes have maximum ESR specifications as low as

5mΩ. In general the lower the ESR specification, the larger

the size and the higher the price. Polytantalum capacitors

have better surge capability than older types and generally

lower ESR. Some types such as the Sanyo TPE and TPB

series have ESR specifications in the 20mΩ to 50mΩ

range, which provide near optimum transient response.

Aluminum Electrolytic Capacitors

Aluminum electrolytic capacitors can also be used with the

LT1764. These capacitors can also be used in conjunction

with ceramic capacitors. These tend to be the cheapest

and lowest performance type of capacitors. Care must be

used in selecting these capacitors as some types can have

ESR which can easily exceed the 3Ω maximum value.

1764afb

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