Datasheet

P1
1
f =
2 x 6 M x C3p W
P2
2
f =
2 x Rout x C2p
2
RHPZ
Rout
f = x
2 x L
Vin
Voutp
æ ö
ç ÷
è ø
Z
1
f =
2 x R3 x C3p
1.229
A =
Vout
Vin 1
x Gea x 6 M x x Rout x
Vout x Rsense 2
W
C
out
+
ǒ
V
out
* V
in
Ǔ
I
out
V
out
Fs V
ripple
V
ripple_ESR
+ I
out
R
ESR
TPS61170-Q1
www.ti.com SLVSAX2 – SEPTEMBER 2011
SCHOTTKY DIODE SELECTION
The high switching frequency of the TPS61170-Q1 demands a high-speed rectifying switch 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-Q1. However, Schottky diodes with lower rated
voltages can be used for lower output voltages to save the solution size and cost. For example, a converter
providing a 12V output with 20V diode is a good choice.
COMPENSATION CAPACITOR SELECTION
The TPS61170-Q1 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 capacitor C3 are connected to COMP pin to provide a pole and a zero. This pole and zero, along with
the inherent pole of 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 of a TPS61170-Q1 boost converter with
ceramic output capacitor (C2), 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).
(7)
(8)
(9)
(10)
(11)
where Rout is the load resistance, Gea is the error amplifier transconductance located in the ELECTRICAL
CHARACTERISTICS table, Rsense (100mΩ typical) is a sense resistor in the current control loop. These
equations helps generate a simple bode plot for TPS61170-Q1 loop analysis.
Increasing R3 or reducing C3 increases the close loop bandwidth which improves the transient response.
Adjusting R3 and C3 in opposite directions increase the phase, and help loop stability. For many 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 SLVA319 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. The
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.
(12)
Where, V
ripple
= peak-to-peak output ripple. The additional output ripple component caused by ESR is calculated
using:
(13)
Due to its low ESR, Vripple_ESR can be neglected for ceramic capacitors, but must be considered if tantalum or
electrolytic capacitors are used.
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Product Folder Link(s): TPS61170-Q1