LP3907_14 Datasheet, PDF (43/57 Pages) Texas Instruments – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C-Compatible Interface

LP3907
www.ti.com
SNVS511N – JUNE 2007 – REVISED AUGUST 2011
The ceramic capacitor’s capacitance can vary with temperature. The capacitor type X7R, which operates over a
temperature range of −55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R
has a similar tolerance over a reduced temperature range of −55°C to +85°C. Many large value ceramic
capacitors, larger than 1µF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance
can drop by more than 50% as the temperature varies from 25°C to 85°C. Therefore X7R is recommended over
Z5U and Y5V in applications where the ambient temperature will change significantly above or below 25°C.
Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more
expensive when comparing equivalent capacitance and voltage ratings in the 0.47µF to 4.7µF range.
Another important consideration is that tantalum capacitors have higher ESR values than equivalent size
ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the
stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic
capacitor with the same ESR value. It should also be noted that the ESR of a typical tantalum will increase about
2:1 as the temperature goes from 25°C down to −40°C, so some guard band must be allowed.
Input Capacitor Selection for SW1 and SW2
A ceramic input capacitor of 10µF, 6.3V is sufficient for the magnetic dc/dc converters. Place the input capacitor
as close as possible to the input of the device. A large value may be used for improved input voltage filtering.
The recommended capacitor types are X7R or X5R. Y5V type capacitors should not be used. DC bias
characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The
input filter capacitor supplies current to the PFET switch of the dc/dc converter in the first half of each cycle and
reduces voltage ripple imposed on the input power source. A ceramic capacitor’s low ESR (Equivalent Series
Resistance) provides the best noise filtering of the input voltage spikes due to fast current transients. A capacitor
with sufficient ripple current rating should be selected. The Input current ripple can be calculated as:
I_rms = I_outmax
V_OUT
V_IN
§ ^r2 ·
© ^{1 +} 12¹^where
r = ^{(Vin ± Vout) x Vout}
L x f x I_outmax x V_in
(7)
The worse case is when V_IN = 2V_OUT.
Output Capacitor Selection for SW1, SW2
A 10μF, 6.3V ceramic capacitor should be used on the output of the sw1 and sw2 magnetic dc/dc converters.
The output capacitor needs to be mounted as close as possible to the output of the device. A large value may be
used for improved input voltage filtering. The recommended capacitor types are X7R or X5R. Y5V type
capacitors should not be used. DC bias characteristics of ceramic capacitors must be considered when selecting
case sizes like 0805 and 0603. DC bias characteristics vary from manufacturer to manufacturer and DC bias
curves should be requested from them and analyzed as part of the capacitor selection process.
The output filter capacitor of the magnetic dc/dc converter smooths out current flow from the inductor to the load,
helps maintain a steady output voltage during transient load changes and reduces output voltage ripple. These
capacitors must be selected with sufficient capacitance and sufficiently low ESD to perform these functions.
The output voltage ripple is caused by the charging and discharging of the output capacitor and also due to its
ESR and can be calculated as follows:
I_ripple
V_pp-c = _{4 x f x C}
(8)
Voltage peak-to-peak ripple due to ESR can be expressed as follows:
V_PP–ESR = 2 × I_RIPPLE × R_ESR
(9)
Because the V_PP-C and V_PP-ESR are out of phase, the rms value can be used to get an approximate value of the
peak-to-peak ripple:
V_pp-rms = V_pp-c2 + V_pp-esr2
(10)
Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series
resistance of the output capacitor (R_ESR). The R_ESR is frequency dependent as well as temperature dependent.
The R_ESR should be calculated with the applicable switching frequency and ambient temperature.
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