Datasheet

( )
2
__
2
_
2
1
_arg DCDCOUTINCDECCOUTLosseCh
VVCE -´´=
( )
DCDCOUTINCDECCOUTCDECCOUT
VVCdQ
____
-´=
TPS62732, TPS62733
TPS62730
SLVSAC3C –MAY 2011–REVISED DECEMBER 2012 www.ti.com
ADDITIONAL DECOUPLING CAPACITORS
In addition to the output capacitor there are further decoupling capacitors connected to the output of the
TPS62730. These decoupling capacitor are placed closely at the RF transmitter or micro controller. The total
capacitance of these decoupling capacitors should be kept to a minimum and should not exceed the values given
in the reference designs, see Figure 31 and Figure 32. During mode transition from DC/DC operation to bypass
mode the capacitors on the output VOUT are charged up to the battery voltage VIN via the internal bypass
switch. During mode transition from bypass mode to DC/DC operation, these capacitors need to be discharged
by the system supply current to the nominal output voltage threshold until the DC/DC will kick in. The charge
change in the output and decoupling capacitors can be calculated according to Equation 6. The energy loss due
to charge/discharge of the output and decoupling capacitors can be calculated according to Equation 7
(6)
(7)
with
dQ
COUT_CDEC
: Charge change needed to charge up / discharge the output and decoupling capacitors from
VOUT_DC_DC to V
IN
and vice versa
C
COUT_CDEC
: Total capacitance on the VOUT pin of the device, includes output and decoupling capacitors
V
IN
: Input (battery) voltage
V
OUT_DC_DC
: nominal DC/DC output voltage V
OUT
INPUT CAPACITOR SELECTION
Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is
required for best input voltage filtering to ensure proper function of the device and to minimize input voltage
spikes. For most applications a 2.2µF to 4.7µF ceramic capacitor is recommended. The input capacitor can be
increased without any limit for better input voltage filtering.
Table 2 shows a list of tested input/output capacitors.
INPUT BUFFER CAPACITOR SELECTION
In addition to the small ceramic input capacitor a larger buffer capacitor C
Buf
is recommended to reduce voltage
drops and ripple voltage. When using battery chemistries like Li-SOCl2, Li-SO2, Li-MnO2, the impedance of the
battery has to be considered. These battery types tend to increase their impedance depending on discharge
status and often can support output currents of only a few mA. Therefore a buffer capacitor is recommended to
stabilize the battery voltage during DC/DC operations e.g. for a RF transmission. A voltage drop on the input of
the TPS62730 during DC/DC operation impacts the advantage of the step down conversion for system power
reduction. Furthermore the voltage drops can fall below the minimum recommended operating voltage of the
device and leads to an early system cut off. Both impacts effects reduce the battery life time. To achieve best
performance and to extract most energy out of the battery a good procedure is to design the select the buffer
capacitor value for an voltage drop below 50mVpp during DC/DC operation. The capacitor value strongly
depends on the used battery type, as well the current consumption during a RF transmission as well the duration
of the transmission.
Table 2. List of Capacitor
CAPACITANCE [μF] SIZE CAPACITOR TYPE SUPPLIER
2.2 0402 GRM155R60J225 Murata
CHECKING LOOP STABILITY
The first step of circuit and stability evaluation is to look from a steady-state perspective at the following signals:
• Switching node, SW
• Inductor current, I
L
• Output ripple voltage, V
OUT(AC)
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