Datasheet
Vout
1
1
Vin
Vout = Vout + ESR
L 8 Cout
-
æ ö
D ´ ´
ç ÷
´ ¦ ´ ´ ¦
è ø
SET SET
CHARGE SET
SET SET
K K
I = R =
R I
TPS65720
TPS65721
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SLVS979 –OCTOBER 2009
(5)
Where the highest output voltage ripple occurs at the highest input voltage Vin.
At light load currents the converter operates in Power Save Mode and the output voltage ripple is dependent on
the output capacitor value. The output voltage ripple is set by the internal comparator delay and the external
capacitor. The typical output voltage ripple is less than 1% of the nominal output voltage.
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 and minimizing the interference with other circuits caused by high input
voltage spikes. The converters need a ceramic input capacitor of 4.7μF. The input capacitor can be increased
without any limit for better input voltage filtering.
Table 2. Tested Capacitors
TYPE VALUE VOLTAGE RATING SIZE SUPPLIER MATERIAL
GRM155R60G475ME47D 4.7 μF 4 V 0402 Murata Ceramic X5R
GRM155R60J225ME15D 2.2 μF 6.3 V 0402 Murata Ceramic X5R
GRM188R60J475K 4.7 μF 6.3 V 0603 Murata Ceramic X5R
GMK107BJ105K 1 μF 35 V 0603 Taiyo Yuden Ceramic X5R
CHARGER/POWER PATH
Charger Stability
In order to ensure stable operation of the charger including the power path, a list of components and their
recommended value is given below. Note that these values represent the capacitance or inductance value in the
application under the given operating conditions. For example, ceramic capacitors will typically show a drop in
capacitance when a dc voltage is applied. Due to this dc bias effect, the capacitance in the applications when
voltage is applied is much less than the nominal capacitor value. See the manufacturers data sheet on this.
At pin AC, a series inductance of may be used with a values as stated below.
Pins AC, SYS and BAT have been tested to be stable with the values given in the table:
PIN NAME Cmin (μF) Cmax (μF) Lmin (μH) lmax (μH)
AC 0.1 1 0 2
SYS 1 10 – –
BAT 0.1 4.7 – –
Setting the Charge Current
The charge current is set with an external resistor connected form ISET to GND.
The resulting charge current is:
(6)
Additionally, the charge current can be scaled to 100%, 75%, 50% or 25% of the value set by Rset by software in
register CHCONFIG1 using Bits ICH_SCL[1,0]. Pre-charge current and termination current is scaled accordingly.
Dynamic Power Path Management (DPPM)
The charger/power path in TPS6572x contains two different features to ensure there is sufficient power at the
load and the input voltage supplying the charger/power path does not collapse.
First there is output voltage DPPM, which is a control loop to keep the voltage at the output of the power path
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