LT3015EMSE データシートの表示（PDF） - Linear Technology
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LT3015EMSE Datasheet PDF : 26 Pages
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LT3015 Series
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
VOUT
R1
R2
(V)
(kΩ)
(kΩ)
–2.5
12.1
12.7
–3.0
12.1
17.8
–3.3
12.1
20.5
–5.0
12.1
37.4
–12.0
12.1
107
–15.0
12.4
140
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 (CFF) from VOUT to VADJ.
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 220µVRMS to 70µVRMS when the output
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 (CFF) from VOUT to VADJ has
the added benefit of improving transient response and PSRR
foroutputvoltagesgreaterthan–1.22V. Withnofeedforward
capacitor, the response and settling times will increase as
the output voltage is raised above –1.22V. Use the equa-
tion in Figure 3 to determine the minimum value of CFF to
achieve a transient (and noise) performance that is similar
16
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.
GND
CIN
R1
COUT
LT3015
SHDN
VIN
IN
ADJ
R2
OUT
CFF
VOUT
3015 F03
CFF ≥ 10nF/100µA • IFB-DIVIDER
IFB-DIVIDER = VOUT/(R1+R2)
Figure 3. Feedforward Capacitor for Fast Transient
Response, Low Noise, and High PSRR
Output Capacitance and Transient Performance
The LT3015 regulators are stable with a wide range of
output capacitors. The ESR of the output capacitor affects
stability, most notably with small capacitors. Use a mini-
mum 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 val-
ues of output capacitance decrease the peak deviations
and provide improved 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 4 and 5.
3015fb
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