Datasheet

DV
ESR
+ I
OUT
R
ESR
C
min
+
I
OUT
V
OUT
f
S
DV
ǒ
V
OUT
* V
IN
Ǔ
TPS63700
www.ti.com SLVS530C –SEPTEMBER 2005–REVISED JUNE 2013
The following inductor series from different suppliers have been tested with the TPS63700 converter:
Table 3. List of Inductors
Output Voltage Vendor SUGGESTED INDUCTOR
VLF4012 4.7 μH
–5V TDK
SLF6025-4.7 μH
LPS4018 4.7 μH
–5V Coilcraft
LPS3015 4.7 μH
–12V Sumida CDRH5D18 10 μH
–12V Coilcraft MOS6020 10 μH
Capacitor Selection
Input Capacitor
At least a 10-μF ceramic input capacitor is recommended for a good transient behavior of the regulator, and EMI
behavior of the total power supply circuit.
Output Capacitors
One of the major parameters necessary to define the capacitance value of the output capacitor is the maximum
allowed output voltage ripple of the converter. This ripple is determined by two parameters of the capacitor, the
capacitance and the ESR. It is possible to calculate the minimum capacitance needed for the defined ripple,
supposing that the ESR is zero, by using Equation 6 for the inverting converter output capacitor.
(6)
Parameter f is the switching frequency and ΔV is the maximum allowed ripple.
With a chosen ripple voltage in the range of 10 mV, a minimum capacitance of 12 μF is needed. The total ripple
is larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using
Equation 7 .
(7)
An additional ripple of 2 mV is the result of using a typical ceramic capacitor with an ESR in a 10-mΩ range. The
total ripple is the sum of the ripple caused by the capacitance, and the ripple caused by the ESR of the capacitor.
In this example, the total ripple is 12 mV. Additional ripple is caused by load transients. When the load current
increases rapidly, the output capacitor must provide the additional current until the inductor current has been
increased by the control loop by setting a higher on-time at the main switch (duty cycle). The higher duty cycle
results in longer inductor charging periods. But the rate of increase of the inductor current is also limited by the
inductance itself. When the load current decreases rapidly, the output capacitor needs to store the excessive
energy (stored in the inductor) until the regulator has decreased the inductor current by reducing the duty cycle.
The recommendation is to use higher capacitance values, as the previous calculations show.
Stabilizing the Control Loop
Feedback Divider
To speed up the control loop, a feed-forward capacitor of 10 pF is recommended in the feedback divider, parallel
to R3.
To avoid coupling noise into the control loop from the feed-forward capacitor, the feed-forward effect can be
bandwidth-limited by adding series resistor R4. A value in the range of 100 kΩ is suitable. The higher the
resistance, the lower the noise coupled into the control loop system.
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