Datasheet

TRAN
O
UT
LOOP-BW TRAN
ΔI
C =
2 f ΔV´ p ´ ´
_
+
(1-D)
R
SENSE
R1
C2
R
2
O
R
ESR
R2
V
ref
C5
C4
R3
(optional)
TPS61175
SLVS892B –DECEMBER 2008–REVISED FEBRUARY 2012 www.ti.com
Due to its low ESR, Vripple_ESR can be neglected for ceramic capacitors, but must be considered if tantalum or
electrolytic capacitors are used.
The minimum ceramic output capacitance needed to meet a load transient requirement can be estimated by the
equation below:
(9)
Where
ΔI
TRAN
is the transient load current step
ΔV
TRAN
is the allowed voltage dip for the load current step
f
LOOP-BW
is the control loop bandwidth (i.e., the frequency where the control loop gain crosses zero).
Care must be taken when evaluating a ceramic capacitor’s derating under dc bias, aging and AC signal. For
example, larger form factor capacitors (in 1206 size) have their self resonant frequencies in the range of the
switching frequency. So the effective capacitance is significantly lower. The DC bias can also significantly reduce
capacitance. Ceramic capacitors can loss as much as 50% of its capacitance at its rated voltage. Therefore, one
must add margin on the voltage rating to ensure adequate capacitance at the required output voltage.
For a typical boost converter implementation, at least 4.7μF of ceramic input and output capacitance is
recommended. Additional input and output capacitance may be required to meet ripple and/or transient
requirements.
The popular vendors for high value ceramic capacitors are:
TDK (http://www.component.tdk.com/components.php)
Murata (http://www.murata.com/cap/index.html)
COMPENSATING THE SMALL SIGNAL CONTROL LOOP
All continuous mode boost converters have a right half plane zero (ƒ
RHPZ
) due to the inductor being removed
from the output during charging. In a traditional voltage mode controlled boost converter, the inductor and output
capacitor form a small signal double pole. For a negative feedback system to be stable, the fed back signal must
have a gain less than 1 before having 180 degrees of phase shift. With its double pole and RHPZ all providing
phase shift, voltage mode boost converters are a challenge to compensate. In a converter with current mode
control, there are essentially two loops, an inner current feedback loop created by the inductor current
information sensed across R
SENSE
(40mΩ) and the output voltage feedback loop. The inner current loop allows
the switch, inductor and modulator to be lumped together into a small signal variable current source controlled by
the error amplifier, as shown in Figure 15.
Figure 15. Small Signal Model of a Current Mode Boost in CCM
The new power stage, including the slope compensation, small signal model becomes:
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