LTC3245 Datasheet, PDF (11/16 Pages) Linear Technology – Wide VIN Range, Low Noise, 250mA Buck-Boost Charge Pump

LTC3245
Applications Information
strength. The value of C_OUT directly controls the amount
of output ripple for a given load current when operating
in constant frequency mode. Increasing the size of C_OUT
will reduce the output ripple.
To reduce output noise and ripple, it is suggested that a
low ESR (equivalent series resistance < 0.1Ω) ceramic
capacitor (10μF or greater) be used for C_OUT. Tantalum
and aluminum capacitors can be used in parallel with a
ceramic capacitor to increase the total capacitance but
are not recommended to be used alone because of their
high ESR.
Both the style and value of C_OUT can significantly affect the
stability of the LTC3245. As shown in the Block Diagram,
the device uses a control loop to adjust the strength of the
charge pump to match the current required at the output.
The error signal of this loop is stored directly on the output
charge storage capacitor. The charge storage capacitor also
serves to form the dominant pole for the control loop. To
prevent ringing or instability it is important for the output
capacitor to maintain at least 4μF of capacitance over all
conditions (see Ceramic Capacitor Selection Guidelines).
Likewise excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3245. The closed
loop output resistance of the device is designed to be 0.3Ω
for a 5V output and 0.2Ω for a 3.3V output. For a 250mA
load current change, the output voltage will change by
about 1.5%V. If the output capacitor has more ESR than
the closed loop impedance, the closed loop frequency
response will cease to roll off in a simple 1-pole fashion
and poor load transient response or instability could result.
Ceramic capacitors typically have exceptional ESR perfor-
mance, and combined with a tight board layout, should
yield excellent stability and load transient performance.
V_IN Capacitor Selection
The constant frequency architecture used by the LTC3245
makes input noise filtering much less demanding than with
conventional regulated charge pumps. Depending on the
mode of operation the input current of the LTC3245 can
vary from I_OUT to 0mA on a cycle-by-cycle basis. Low ESR
will reduce the voltage steps caused by changing input
current, while the absolute capacitor value will determine
the level of ripple. The total amount and type of capacitance
necessary for input bypassing is very dependant on the
applied source impedance as well as existing bypassing
already on the V_IN node. For optimal input noise and ripple
reduction, it is recommended that a low ESR ceramic
capacitor be used for C_IN bypassing. An electrolytic or
tantalum capacitor may be used in parallel with the ce-
ramic capacitor on C_IN to increase the total capacitance,
but due to the higher ESR it is not recommended that an
electrolytic or tantalum capacitor be used alone for input
bypassing. The LTC3245 will operate with capacitors less
than 1μF but depending on the source impedance input
noise can feed through to the output causing degraded
performance. For best performance 1μF or greater total
capacitance is suggested for C_IN.
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or alumi-
num should never be used for the flying capacitors since
the voltage can reverse upon start-up of the LTC3245.
Ceramic capacitors should always be used for the flying
capacitors. The flying capacitors control the strength of
the charge pump. In order to achieve the rated output
current, it is necessary for the flying capacitor to have
at least 0.4μF of capacitance over operating temperature
with a bias voltage equal to the programmed V_OUT (see
Ceramic Capacitor Selection Guidelines). If only 100mA
or less of output current is required for the application,
the flying capacitor minimum can be reduced to 0.15μF.
The voltage rating of the ceramic capacitor should be
V_OUT + 1V or greater.
Ceramic Capacitor Selection Guidelines
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates.
For example, a ceramic capacitor made of X5R or X7R
material will retain most of its capacitance from –40°C
to 85°C, whereas a Z5U or Y5V style capacitor will lose
considerable capacitance over that range (60% to 80%
loss typical). Z5U and Y5V capacitors may also have a
very strong voltage coefficient, causing them to lose an
additional 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
3245f
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