lt3015edd Linear Technology Corporation, lt3015edd Datasheet - Page 12

• Manufacturer Part Number: lt3015edd
• Description: 1.5a, Low Noise, Negative Linear Regulator With Precision Current Limit
• Manufacturer: Linear Technology Corporation

LT3015
APPLICATIONS INFORMATION
Table 1 shows 1% resistor divider values for some com-
mon output voltages with a resistor divider current of
approximately 100μA.
Table 1. Output Voltage Resistor Divider Values
Feedforward Capacitance: Output Voltage Noise,
Transient Performance, and PSRR
The LT3015 regulators provide low output voltage noise
over the 10Hz to 100kHz bandwidth while operating at
full load current. Output voltage noise is approximately
240nV/√Hz over this frequency while operating in unity-gain
configuration. For higher output voltages (using a resistor
divider), the output voltage noise gains up accordingly. To
lower the output voltage noise for higher output voltages,
include a feedforward capacitor (C
A good quality, low leakage, capacitor is recommended.
This capacitor bypasses the resistor divider network at high
frequencies; and hence, reduces the output noise. With
the use of a 10nF feedforward capacitor, the output noise
decreases from 220μV
voltage is set to –5V by a 100μA feedback resistor divider.
Higher values of output voltage noise are often measured
if care is not exercised with regard to circuit layout and
testing. Crosstalk from nearby traces induces unwanted
noise onto the LT3015’s output. Moreover, power supply
ripple rejection (PSRR) must also be considered, as the
LT3015 does not exhibit unlimited PSRR; and thus, a
small portion of the input noise propagates to the output.
Using a feedforward capacitor (C
the added benefit of improving transient response and PSRR
for output voltages greater than –1.22V. With no feedforward
capacitor, the response and settling times will increase as
the output voltage is raised above –1.22V. Use the equa-
tion in Figure 2 to determine the minimum value of C
achieve a transient (and noise) performance that is similar
12
–12.0
–15.0
V
–2.5
–3.0
–3.3
–5.0
(V)
OUT
RMS
(kΩ)
12.1
12.1
12.1
12.1
12.1
12.4
R1
to 70μV
FF
) from V
FF
RMS
) from V
when the output
OUT
OUT
(kΩ)
12.7
17.8
20.5
37.4
107
140
to V
R2
to V
ADJ
FF
ADJ
has
to
.
to –1.22V output voltage performance regardless of the
chosen output voltage (see Transient Response and Output
Noise in the Typical Performance Characteristics section).
It is important to note that the start-up time is affected by
the use of a feedforward capacitor. Start-up time is directly
proportional to the size of the feedforward capacitor and
the output voltage, and is inversely proportional to the
feedback resistor divider current. In particular, it slows
to 860μs with a 10nF feedforward capacitor and a 10μF
output capacitor for an output voltage set to –5V by a
100μA feedback resistor divider current.
Output Capacitance and Transient Performance
The LT3015 regulator is stable with a wide range of output
capacitors. The ESR of the output capacitor affects stabil-
ity, most notably with small capacitors. Use a minimum
output capacitor of 10μF with an ESR of 500mΩ or less to
prevent oscillations. The LT3015’s load transient response
is a function of output capacitance. Larger values of output
capacitance decrease the peak deviations and provide im-
proved transient response for larger load current changes.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used are specified with EIA temperature char-
acteristic codes of Z5U, Y5V, X5R, and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but they tend to have strong voltage
and temperature coefficients as shown in Figures 3 and 4.
When used with a 5V regulator, a 16V 10μF Y5V capacitor
Figure 2. Feedforward Capacitor for Fast Transient
Response, Low Noise, and High PSRR
V
IN
C
IN
C
I
FB-DIVIDER
FF
SHDN
IN
LT3015
= V
GND
OUT
/(R1+R2)
FB-DIVIDER
OUT
ADJ
3015 F02
R1
R2
C
FF
C
V
OUT
OUT
3015f