LM2770_15 Datasheet, PDF (13/20 Pages) Texas Instruments – High Efficiency Switched Capacitor Step-Down DC/DC Regulator with Sleep Mode

LM2770
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SNVS318E – NOVEMBER 2004 – REVISED MAY 2013
Due to the PFM nature of the LM2770, output voltage ripple is highest at light loads. To eliminate this ripple,
consider running the LM2770 in sleep mode when load currents are 20mA or less. Sleep mode disables the
charge pump and enables the internal low-noise bypass linear regulator (LDO).
INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE
The input capacitor (C_IN) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying
capacitors during the charge phase of operation. The input capacitor helps to keep the input voltage from
drooping at the start of the charge phase when the flying capacitor is connected to the input. It also filters noise
on the input pin, keeping this noise out of sensitive internal analog circuitry that is biased off the input line.
Much like the relationship between the output capacitance and output voltage ripple, input capacitance has a
dominant and first-order effect on input ripple magnitude. Increasing (decreasing) the input capacitance will result
in a proportional decrease (increase) in input voltage ripple. This can be observed in the input voltage ripple
waveforms in the Typical Performance Characteristics section. Input voltage, output current, and flying
capacitance also will affect input ripple levels to some degree.
In typical high-current applications, a 10µF low-ESR ceramic capacitor is recommended on the input. Different
input capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the
solution. But changing the input capacitor may also require changing the flying capacitors and/or output capacitor
to maintain good overall circuit performance. Performance of the LM2770 with different capacitor setups is
discussed below in RECOMMENDED CAPACITOR CONFIGURATIONS.
FLYING CAPACITORS
The flying capacitors (C₁ and C₂) transfer charge from the input to the output. Flying capacitance can impact both
output current capability and ripple magnitudes. If flying capacitance is too small, the LM2770 may not be able to
regulate the output voltage when load currents are high. On the other hand, if the flying capacitance is too large,
the flying capacitors might overwhelm the input and output capacitors, resulting in increased input and output
ripple.
The flying capacitors should be identical. As a general guideline, the capacitance value of each flying capacitor
should be 1/10th that of the output capacitor, up to a maximum of 1µF. This is a recommendation, not a
requirement. Polarized capacitors (tantalum, aluminum electrolytic, etc.) must not be used for the flying
capacitors, however, as they could become reverse-biased during LM2770 operation.
RECOMMENDED CAPACITOR CONFIGURATIONS
The data in Table 1 can be used to assist in the selection of a capacitor configuration that best balances solution
size and cost with the electrical requirements of the application (ripple voltages, output current capability, etc.).
As previously discussed, input and output ripple voltages and frequencies will vary considerably with output
current and input voltage. The numbers provided show expected ripple voltage when V_IN = 3.6V and load
currents are between 100mA and 250mA. The table offers first look at approximate ripple levels and provides a
comparison for the different capacitor configurations presented, but is not intended to ensure performance.
The columns that provide minimum input voltage recommendations illustrate the effect that smaller flying
capacitors have on charge pump output current capability. Using smaller flying capacitors increases the output
resistance of the charge pump. As a result, the minimunm input voltage of an application using small flying
capacitance may need to be set slightly higher to prevent the output from falling out of regulation when loaded.
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