LTC3260_15 Datasheet, PDF (12/18 Pages) Linear Technology – Low Noise Dual Supply Inverting Charge Pump

LTC3260
Applications Information
also have a poor voltage coefficient causing them to lose
60% or more of their capacitance when the rated voltage
is applied. Therefore when comparing different capacitors,
it is often more appropriate to compare the amount of
achievable capacitance for a given case size rather than
discussing the specified capacitance value. The capacitor
manufacture’s data sheet should be consulted to ensure
the desired capacitance at all temperatures and voltages.
Table 1 is a list of ceramic capacitor manufacturers and
their websites.
Table 1
AVX
Kemet
Murata
Taiyo Yuden
Vishay
TDK
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.component.tdk.com
Layout Considerations
Due to high switching frequency and high transient currents
produced by LTC3260, careful board layout is necessary
for optimum performance. A true ground plane and short
connections to all the external capacitors will improve
performance and ensure proper regulation under all condi-
tions. Figure 5 shows an example layout for the LTC3260.
V_IN
LDO⁺
GND
C_FLY
C_BYP–
C_BYP+
RT
GND
Figure 5. Recommended Layout
V_OUT
LDO^–
3260 F05
The flying capacitor nodes C⁺ and C^– switch large cur-
rents at a high frequency. These nodes should not be
routed close to sensitive pins such as the LDO feedback
pins (ADJ⁺ and ADJ^–) and internal reference bypass pins
(BYP⁺ and BYP^–).
Thermal Management
At high input voltages and maximum output current, there
can be substantial power dissipation in the LTC3260. If
the junction temperature increases above approximately
175°C, the thermal shutdown circuitry will automatically
deactivate the output. To reduce the maximum junction
temperature, a good thermal connection to the PC board
ground plane is recommended. Connecting the exposed pad
of the package to a ground plane under the device on two
layers of the PC board can reduce the thermal resistance
of the package and PC board considerably.
Derating Power at High Temperatures
To prevent an overtemperature condition in high power
applications, Figure 6 should be used to determine the
maximum combination of ambient temperature and power
dissipation.
The power dissipated in the LTC3260 should always fall
under the line shown for a given ambient temperature. The
power dissipated in the LTC3260 has three components.
Power dissipated in the positive LDO:
P_LDO+ = (V_IN – V_LDO+) • I_LDO+
Power dissipated in the negative LDO:
P_LDO– = (|V_OUT| – |V_LDO–|) • I_LDO– and
Power dissipated in the inverting charge pump:
P_CP = (V_IN – |V_OUT|) • (I_OUT + I_LDO–)
where I_OUT denotes any additional current that might be
pulled directly from the V_OUT pin. The LDO^– current is
also supplied by the charge pump through V_OUT and is
therefore included in the charge pump power dissipation.
The total power dissipation of the LTC3260 is given by:
P_D = P_LDO+ + P_LDO– + P_CP
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