TPS61170 Datasheet, PDF (16/22 Pages) Texas Instruments – 1.2A High Voltage Boost Converter in 2x2mm2 QFN Package

TPS61170
SLVS789 – NOVEMBER 2007
www.ti.com
SCHOTTKY DIODE SELECTION
The high switching frequency of the TPS61170 demands a high-speed rectification for optimum efficiency.
Ensure that the diode’s average and peak current rating exceeds the average output current and peak inductor
current. In addition, the diode’s reverse breakdown voltage must exceed the switch FET rating voltage of 40V.
So, the ONSemi MBR0540 is recommended for TPS61170. However, Schottky diode of low rating voltage can
be used for low output to save the solution size and cost. For example, 12V output with 20V diode is a good
choice.
COMPENSATION CAPACITOR SELECTION
The TPS61170 has an external compensation, COMP pin, which allows the loop response to be optimized for
each application. The COMP pin is the output of the internal error amplifier. An external resistor R3 and ceramic
capacitors C3 are connected to COMP pin to provide a pole and a zero. This pole and zero, along with the
inherent pole an zero in a current mode control boost converter, determine the close loop frequency response.
This is important to a converter stability and transient response.
The following equations summarize the poles, zeros and DC gain in TPS61170, as shown in the block diagram.
They include the dominant pole (f_P1), the output pole (f_P2) of a boost converter, the right-half-plane zero (f_RHPZ) of
a boost converter, the zero (f_Z) generated by R3 and C3, and the DC gain (A).
1
f_P1 = _{2p x 6 MW x C3}
(7)
2
f_P2 = _{2p x Rout x C2}
(8)
Rout æ Vin ö²
f_RHPZ = _{2p x L} x _{çè Vout ÷ø}
(9)
1
f_Z = _{2p x R3 x C3}
(10)
1.229
Vin
1
A=
x Gea x 6 MW x
x Rout x
Vout
Vout x Rsense
2
(11)
where Rout is the load resistance, Gea is the error amplifier transconductance located in the ELECTRICAL
CHARACTERISTICS table, Rsense (100mΩ) is a sense resistor in the current control loop. These equations
helps generate a simple bode plot for TPS61170 loop analysis.
Increasing R3 or reducing C3 increases the close loop bandwidth which improves the transient response.
Adjusting R3 and C3 toward opposite direction increase the phase, and help loop stability. For most of the
applications, the recommended value of 10k and 680pF makes an ideal compromise between transient response
and loop stability. To optimize the compensation, use C3 in the range of 100pF to 10nF, and R3 of 10k. See the
TI application report for thorough analysis and description of the boost converter small signal model and
compensation design.
INPUT AND OUTPUT CAPACITOR SELECTION
The output capacitor is mainly selected to meet the requirements for the output ripple and loop stability. This
ripple voltage is related to the capacitor’s capacitance and its equivalent series resistance (ESR). Assuming a
capacitor with zero ESR, the minimum capacitance needed for a given ripple can be calculated using
Equation 12.
C_out
+
ǒ_Vout
V_out
* V_in^Ǔ I_out
Fs V_ripple
(12)
Where, V_ripple = peak-to-peak output ripple. The additional output ripple component caused by ESR is calculated
using:
V_{ripple_ESR} + I_out R_ESR
Due to its low ESR, Vripple_ESR can be neglected for ceramic capacitors, but must be considered if tantalum or
electrolytic capacitors are used.
16
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