Datasheet

LM2757
www.ti.com SNVS536E –OCTOBER 2007–REVISED MAY 2013
In typical high-current applications, 0.47µF low-ESR ceramic capacitors are recommended for the flying
capacitors. Polarized capacitors (tantalum, aluminum electrolytic, etc.) must not be used for the flying capacitor,
as they could become reverse-biased during LM2757 operation.
Recommended Capacitance
The data in Table 1 can be used to assist in the selection of capacitance for each node that best balances
solution size and cost with the electrical requirements of the application.
As previously discussed, input and output ripple voltages will vary with output current and input voltage. The
numbers provided show expected ripple voltage with V
IN
= 3.6V and a load current of 200mA at 5V output,
100mA at 4.5V output, and 100mA at 4.1V output. The table offers a first look at approximate ripple levels and
provides a comparison of different capacitance configurations, but is not intended to ensure performance. With
any capacitance configuration chosen, always verify that the performance of the ripple waveforms are suitable for
the intended application. The same capacitance value must be used for all the flying capacitors. For output
regulation performance with different capacitor configurations, please refer to the Output Voltage vs. Input
Voltage graphs in the Typical Performance Characteristics section of this datasheet. The output voltage
regulation is typically better when using capacitors with a higher capacitance value and a higher voltage bias
rating than the nominal voltage applied to them, as can be seen in the graphs, but this may have an impact in
capacitor case size. For typical high-current small solution size applications, 1µF capacitance X5R temperature
characteristic rating 0402 (C1005) case size and 10V bias or higher capacitors can be used for the Input, Output
and Flying capacitors. According to current capacitor offerings, there are no capacitors in the 0201 (C0603) case
size that satisfy the minimum capacitance requirements of the LM2757 circuit. When selecting capacitors, it is
preferred to use the ones with the highest voltage bias rating available from the capacitor supplier.
Table 1. LM2757 Performance with Different Capacitor Configurations
(1)
Typical Typical Typical
Capacitor
5.0V,200mA 4.5V,100mA 4.1V,100mA
Configuration
Output Output Output
(V
IN
= 3.6V)
Ripple Ripple Ripple
C
IN
= 1µF,
C
OUT
= 1µF, 32mV 12mV 11mV
C
1
, C
2
= 0.47µF
C
IN
= 0.68µF,
C
OUT
= 1µF, 32mV 11mV 11mV
C
1
, C
2
= 0.47µF
C
IN
= 0.68µF,
C
OUT
= 0.47µF, 51mV 15mV 15mV
C
1
, C
2
= 0.47µF
C
IN
= 0.68µF,
C
OUT
= 0.47µF, 53mV 18mV 18mV
C
1
, C
2
= 0.22µF
(1) Refer to the text in the Recommended Capacitance section for detailed information on the data in this table
Layout Guidelines
Proper board layout will help to ensure optimal performance of the LM2757 circuit. The following guidelines are
recommended:
• Place capacitors as close to the LM2757 as possible, and preferably on the same side of the board as the IC.
• Use short, wide traces to connect the external capacitors to the LM2757 to minimize trace resistance and
inductance.
• Use a low resistance connection between ground and the GND pin of the LM2757. Using wide traces and/or
multiple vias to connect GND to a ground plane on the board is most advantageous.
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