Datasheet

( )
I × V - V
IN
OUT OUT
C =
min
f × ΔV × V
OUT
D
D
I× t
Capacitance =
V
1.3A ×1.154ms
Capacitance = = 4.2mF
350mV
TPS62750, TPS62751
www.ti.com ................................................................................................................................................. SLVS955A –JULY 2009–REVISED SEPTEMBER 2009
OUTPUT CAPACITOR SELECTION
The TPS62750 has been specifically internally compensated to operate with large capacitance values. But to
maintain loop stability of the device, it is recommended to use a small ceramic capacitor placed as close as
possible to the V
OUT
and GND pins of the IC in parallel with the large holdup capacitor. To get an estimate of the
small ceramic recommended minimum output capacitance, Equation 4 can be used.
(4)
Parameter f is the switching frequency and ΔV is the maximum allowed ripple.
With a chosen ripple voltage of 10 mV, a minimum effective capacitance of 2.7 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 Δ
V
ESR
= I
OUT
x R
ESR
.
A capacitor with a value in the range of the calculated minimum should be used. This is required to maintain
control loop stability. There are no additional requirements regarding minimum ESR. There is no upper limit for
the output capacitance value. Larger capacitors cause lower output voltage ripple as well as lower output voltage
drop during load transients.
Note that ceramic capacitors have a DC Bias effect, which will have a strong influence on the final effective
capacitance needed. Therefore the right capacitor value has to be chosen very carefully. Package size and
voltage rating in combination with material are responsible for differences between the rated capacitor value and
the effective capacitance.
To calculate the value of the effective capacitance required to buffer a GSM transmission pulse, the following
equations can be used:
(5)
Assuming the DCDC supplies ~700mA, the rest of the energy to supply the GSM transmission pulse must come
from the capacitor.
I
CAP
= I
GSM
- I
DCDC
I
CAP
= 2A - 700mA = 1.3A
Assuming a GSM transmission pulse width of 1.154ms and allowing a maximum voltage drop on the output of
350mV, the effective capacitance required is:
(6)
Table 4. List of Capacitors
COMPONENT REFERENCE PART NUMBER MANUFACTURER VALUE
C
O
GRM188R60J106M69D Murata 10μF
6TPG150M Sanyo POSCAP 150μF
C
BULK
592D158X06R3X2T25H Vishay 1.5mF
592D228X06R3X2T22H Vishay 2.2mF
AVERAGE INPUT CURRENT LIMIT
The average input current is set by selecting the correct external resistor value correlating to the required current
limit. The current limit can be selected between a high current limit (I
LIM_U
) and a lower limit (I
LIM_L
) by toggling
the H/L pin high or low. This has the added benefit that of allowing a device first plugged into the USB port to
enumerate at 100mA before switching over to the high power mode (500mA). The equations below are a
guideline for selecting the correct resistor value:
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