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TCL1585

7A / 5A / 4.6A / 3A, FAST RESPONSE, LOW DROPOUT POSITIVE LINEAR VOLTAGE REGULATORS

TelCom Semiconductor Inc => Microchip 

7A / 5A / 4.6A / 3A, FAST RESPONSE,

LOW DROPOUT POSITIVE LINEAR

VOLTAGE REGULATORS

PRELIMINARY INFORMATION

TCL1584

TCL1585

TCL1587

APPLICATIONS

age response to step load current change is illustrated in

General

Figure 1. The capacitor’s ESR and ESL cause immediate

step changes in the output voltage. These are calculated as

The TCL158x family of devices combine high current follows:

output (up to 7A) with low dropout voltage and built-in fault

protection in a traditional three-terminal LDO format. All

three device types are available in fixed output voltage and

adjustable output versions. Fault protection includes short-

circuit current limit, over-temperature limit, and safe-operat-

ing-area (SOA) governing.

These devices are pin-compatible upgrades for the

1083/1084/1085 family of LDO’s. However, the TCL158x

family delivers lower dropout voltage, faster load transient

response and improved internal frequency compensation.

Maximum supply voltage rating is 7.0V.

Modern processors cycle load current from near zero to

∆VESR = ∆I x ESR

DVESL = DI/Dt x ESL

To reduce the initial voltage droop, one should select

low ESR and ESL capacitors. It should also be noted that the

ESR effect is multiplied by absolute change in load current

while the ESL effect is multiplied by the rate of change in load

current. After the initial voltage drop, the capacitor value

dominates the rate of change in voltage. This rate is calcu-

lated as follows:

several amps in a time period measured in tens of nanosec-

onds. Load step response requirements are worsened by

∆V = ∆t x ∆I/C

tighter output voltage tolerances. The TCL1584/85/87 fam-

ily of regulators meets these stringent requirements without

an obnoxious amount of output capacitance, saving both

board space and cost.

Stability and Transient Response

Like most low dropout voltage regulators, the TCL158x

devices require the use of output capacitors to maintain

stability. Normally a 22µF solid tantalum or a 100µF alumi-

num electrolytic unit will ensure stability over all operating

ESR

EFFECTS

ESL

EFFECTS

CAPACITANCE

EFFECTS

V

SLOPE, t

=

∆I

C

POINT AT WHICH REGULATOR

TAKES CONTROL

conditions. Keep in mind that commercially available ca-

pacitors can have significant non-ideal effects such as

capacitance value tolerance, temperature coefficient, ESR,

Figure 1. Transient Load Voltage Response

ESL. The TCL158x devices are optimized for use with low

ESR (<1Ω) capacitors.

On the adjustable voltage versions, bypassing the ADJ

pin will improve ripple rejection and transient response. This

is discussed in the Ripple Rejection section. This bypassing

increases the required output capacitance value. The previ-

ously suggested minimum values (22µF and 100µF) take

this into account. If no bypassing is used, lower values of

output capacitance may be used.

Transient regulation is directly related to output capaci-

tance value. For applications which require large load cur-

rent step changes, it is recommended that large output

capacitors (>100µF) be used. The value of the output

capacitor can be increased without limit and will only im-

prove transient regulation.

In a typical high-performance microprocessor applica-

tion, the sudden transients can be so fast that the output

decoupling network must handle the sudden current de-

mand until the internal voltage regulator is able to respond.

In this case the non-ideal effects of the output capacitor are

critical in determining the regulator’s response. Output volt-

Typically high quality ceramic and tantalum capacitors

must be used in combination to minimize ESR and maximize

C. This decoupling network must also be placed close to the

microprocessor to reduce ESL (parasitic board trace induc-

tance). If possible, the capacitors should be placed inside

the microprocessor socket cavity. Of course, robust power

and ground planes will also improve performance by reduc-

ing parasitic voltage drops.

The TCL1584 has an adaptive current limiting scheme

where to ensure SOA for the output transistor, the current

limit is reduced for increasing input to output differential. This

means that the TCL1584 exhibits a negative resistance

characteristic under certain conditions. This is a common

technique in LDO design to ensure SOA - especially LDO’s

with high maximum input voltage ratings. This negative

resistance can interact with the external capacitance and

inductance and cause oscillations during current limit. This

effect is highly dependent on system parameters and is

difficult to predict. However this oscillation, if it occurs, will

not damage the regulator and can be ignored if the system

TCL1584/1585/1587-04 6/6/97

5

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