Datasheet

LT3021/LT3021-1.2/
LT3021-1.5/LT3021-1.8
11
3021fc
APPLICATIONS INFORMATION
amounts 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 overshooting 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
OUT
) 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.
To eliminate this problem, the LT3021 incorporates a
no-load or light-load recovery circuit. This circuit is a
voltage-controlled current sink that signifi cantly improves
the light load transient response time by discharging the
output capacitor quickly and then turning off. The cur-
rent 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 voltage
differential: (I
OUT
)(V
IN
– V
OUT
) and
2. GND pin current multiplied by the input voltage:
(I
GND
)(V
IN
).
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 components
listed above.
Figure 2. Ceramic Capacitor DC Bias Characteristics
Figure 3. Ceramic Capacitor Temperature Characteristics
Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3021 F02
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3021 F03
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
1ms/DIVV
OUT
= 1.3V
C
OUT
= 10F
I
LOAD
= 0
3021 F04
1mV/DIV
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