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3021

500mA, Low Voltage, Very Low Dropout Linear Regulator

Linear Technology 

LT3021/LT3021-1.2/

LT3021-1.5/LT3021-1.8

APPLICATIO S I FOR ATIO

20

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

0

X5R

–20

–40

–60

Y5V

–80

–100

0 2 4 6 8 10 12 14 16

DC BIAS VOLTAGE (V)

3021 F02

Figure 2. Ceramic Capacitor DC Bias Characteristics

40

20

0

X5R

–20

–40

Y5V

–60

–80 BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

–100

–50 –25 0 25 50 75

TEMPERATURE (°C)

100 125

3021 F03

Figure 3. Ceramic Capacitor Temperature Characteristics

of noise. A ceramic capacitor produced Figure 4’s trace in

response to light tapping from a pencil. Similar vibration

induced behavior can masquerade as increased output

voltage noise.

No-Load/Light-Load Recovery

A transient load step occurs when the output current

changes from its maximum level to zero current or a very

small load current. The output voltage responds by over-

shooting until the regulator lowers the amount of current it

delivers to the new level. The regulator loop response time

and the amount of output capacitance control the amount of

overshoot. Once the regulator has decreased its output

current, the current provided by the resistor divider (which

sets VOUT) is the only current remaining to discharge the

output capacitor from the level to which it overshot. The

amount of time it takes for the output voltage to recover

easily extends to milliseconds with microamperes of divider

current and a few microfarads of output capacitance.

10

1mV/DIV

VOUT = 1.3V

COUT = 10µF

ILOAD = 0

1ms/DIV

3021 F04

Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor

To eliminate this problem, the LT3021 incorporates a

no-load or light-load recovery circuit. This circuit is a

voltage-controlled current sink that significantly improves

the light load transient response time by discharging the

output capacitor quickly and then turning off. The current

sink turns on when the output voltage exceeds 6% of the

nominal output voltage. The current sink level is then

proportional to the overdrive above the threshold up to a

maximum of approximately 15mA. Consult the curve in

the Typical Performance Characteristics for the No-Load

Recovery Threshold.

If external circuitry forces the output above the no load

recovery circuit’s threshold, the current sink turns on in an

attempt to restore the output voltage to nominal. The

current sink remains on until the external circuitry releases

the output. However, if the external circuitry pulls the

output voltage above the input voltage, or the input falls

below the output, the LT3021 turns the current sink off and

shuts down the bias current/reference generator circuitry.

Thermal Considerations

The LT3021’s power handling capability is limited by its

maximum rated junction temperature of 125°C. The power

dissipated by the device is comprised of two components:

1. Output current multiplied by the input-to-output volt-

age differential: (IOUT)(VIN – VOUT) and

2. GND pin current multiplied by the input voltage:

(IGND)(VIN).

GND pin current is found by examining the GND pin

current curves in the Typical Performance Characteristics.

Power dissipation is equal to the sum of the two compo-

nents listed above.

3021fa

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