LTC3245 Datasheet, PDF (12/16 Pages) Linear Technology – Wide VIN Range, Low Noise, 250mA Buck-Boost Charge Pump

LTC3245
Applications Information
rather than discussing the specified capacitance value. For
example, over rated voltage and temperature conditions,
a 4.7μF, 10V, Y5V ceramic capacitor in an 0805 case may
not provide any more capacitance than a 1μF, 10V, X5R
or X7R available in the same 0805 case. In fact, over bias
and temperature range, the 1μF, 10V, X5R or X7R will
provide more capacitance than the 4.7μF, 10V, Y5V. The
capacitor manufacturer’s data sheet should be consulted
to determine what value of capacitor is needed to ensure
minimum capacitance values are met over operating
temperature and bias voltage. Below is a list of ceramic
capacitor manufacturers and how to contact them:
MANUFACTURER
AVX
Kemet
Murata
Taiyo Yuden
TDK
WEBSITE
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.tdk.com
Layout Considerations
Due to the high switching frequency and transient cur-
rents produced by the LTC3245, careful board layout is
necessary for optimal performance. A true ground plane
and short connections to all capacitors will optimize
performance, reduce noise and ensure proper regulation
over all conditions.
When using the LTC3245 with an external resistor divider
it is important to minimize any stray capacitance to the
ADJ (OUTS/ADJ pin) node. Stray capacitance from ADJ
to C⁺ or C^– can degrade performance significantly and
should be minimized and/or shielded if necessary.
Thermal Management
The on chip power dissipation in the LTC3245 will cause the
junction to ambient temperature to rise at rate of 40°C/W
or more. To reduce the maximum junction temperature, a
good thermal connection to the PC board is recommended.
Connecting the die paddle (Pin 13) with multiple vias to a
large ground plane under the device can reduce the thermal
resistance of the package and PC board considerably. Poor
board layout and failure to connect the die paddle (Pin 13)
to a large ground plane can result in thermal junction to
ambient impedance well in excess of 40°C/W.
Because of the wide input operating range it is possible to
exceed the specified operating junction temperature and
even reach thermal shutdown. Figure 3 shows the avail-
able output current vs temperature to ensure the 150°C
operating junction temperature is not exceed for input
voltages less than 20V.
Figure 3 assumes worst-case operating conditions. Under
some operating conditions the part can supply more current
than shown without exceeding the 150°C operating junc-
tion temperature. When operating outside the constraints
of Figure 3 it is the responsibility of the user to calculate
worst-case operating conditions (temperature and power)
to make sure the LTC3245’s specified operating junction
temperature is not exceeded for extended periods of time.
The 2:1 Step-Down, 1:1 Step-Down, and 1:2 Step-Up
Charge Pump Operation sections provide equations for
calculating power dissipation (P_D) in each mode.
300
V_IN < 20V
250
200
150
100
50
0
70 80 90 100 110 120 130 140 150
TEMPERATURE (°C)
3245 F03
Figure 3. Available Output Current vs Temperature
For example, if it is determined that the maximum power
dissipation (P_D) is 1.2W under normal operation, then the
junction to ambient temperature rise will be:
Junction to ambient = 1.2W • 40°C/W = 48°C
Thus, the ambient temperature under this condition cannot
exceed 102°C if the junction temperature is to remain below
150°C and if the ambient temperature exceeds about 127°C
the device will cycle in and out of the thermal shutdown.
3245f
12
For more information www.linear.com/LTC3245