lt3021es8-trpbf Linear Technology Corporation, lt3021es8-trpbf Datasheet - Page 10

• Manufacturer Part Number: lt3021es8-trpbf
• Description: 500ma, Low Voltage, Very Low Dropout Linear Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT3021/LT3021-1.2/
LT3021-1.5/LT3021-1.8
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 V
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
Figure 3. Ceramic Capacitor Temperature Characteristics
Figure 2. Ceramic Capacitor DC Bias Characteristics
OUT
) is the only current remaining to discharge the
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
40
20
20
0
0
–50
0
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
–25
2
U
4
DC BIAS VOLTAGE (V)
0
TEMPERATURE (°C)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
6
25
U
8
Y5V
X5R
50
10
Y5V
75
12
X5R
W
100
14
3021 F03
3021 F02
125
16
U
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-
2. GND pin current multiplied by the input voltage:
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.
Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor
age differential: (I
(I
GND
1mV/DIV
)(V
IN
V
C
I
LOAD
OUT
OUT
).
= 1.3V
= 10µF
= 0
OUT
)(V
1ms/DIV
IN
– V
OUT
) and
3021 F04
3021fb