TB7102AF Datasheet, PDF (7/17 Pages) Toshiba Semiconductor – Buck DC-DC Converter IC

TB7102AF
L = ^{VIN − VOUT} ⋅ ^VOUT
f_osc ⋅ ΔI_L V_IN
= ^{5 V − 3.3 V} ⋅ ^{3.3 V} ······ (2)
1 MHz ⋅ 300 mA 5 V
= 3.7 μH
I_L
0
T= ¹
f_osc
T_ON
=
Τ
⋅
V_OUT
V_IN
Figure 2 Inductor Current Waveform
Setting the Output Voltage
A resistive voltage divider is connected as shown in Figure 3 to set the output voltage; it is given by Equation 3
based on the reference voltage of the error amplifier, which is connected to the Feedback pin, V_FB. RFB1 should
be up to 10 kΩ or so, because an extremely large value RFB1 incurs a delay due to parasitic capacitance at the
V_FB pin. If the difference between the input and output voltages is small, the output voltage may drop,
depending on the load current conditions. For optimal operation, output voltage should be set to 0.8 V (typ.) at
the minimum and to (V_IN −1) V at the maximum. It is recommended that resistors with a precision of ±1% or
higher be used for R_FB1 and R_FB2.
V_OUT
=
V_FB
⋅ _{⎜⎜⎝⎛}1
+
R_FB1
R_FB2
⎟⎟⎠⎞
=
0.8
V
×
⎜⎜⎝⎛¹
+
R_FB1
R_FB2
⎟⎟⎠⎞
······
(3)
L_X
V_OUT
FB
Figure 3 Output Voltage Setting Resistors
Output Capacitor Selection
Use a ceramic capacitor as the output filter capacitor. Since a ceramic capacitor is generally sensitive to
temperature, choose one with excellent temperature characteristics (such as the JIS B characteristic). As a rule
of thumb, its capacitance should be 10 μF or greater for applications where V_OUT ≥ 2.0 V, and 20 μF or greater
for applications where V_OUT < 2.0 V. The capacitance should be set to an optimal value that meets the system's
ripple voltage requirement and transient load response characteristics. Since the ceramic capacitor has a very
low ESR value, it helps reduce the output ripple voltage; however, because the ceramic capacitor provides less
phase margin, it should be thoroughly evaluated.
Component Values (@V_IN = 5 V, Ta = 25°C)
These values are presented only as a guide.
The following values may need tuning depending on the TB7102AF’s input/output conditions and the board
layout.
Output Voltage
Setting
V_OUT
1.2 V
1.5 V
1.8 V
2.5 V
3.3 V
Inductance
L
3.3 μH
3.3 μH
3.3 μH
3.3 μH
3.3 μH
Input Capacitance Output Capacitance Feedback Resistor Feedback Resistor
C_IN
10 μF
10 μF
10 μF
10 μF
10 μF
C_OUT
22 μF
22 μF
22 μF
10 μF
10 μF
R_FB1
1.2 kΩ
2.1 kΩ
3.0 kΩ
5.1 kΩ
7.5 kΩ
R_FB2
2.4 kΩ
2.4 kΩ
2.4 kΩ
2.4 kΩ
2.4 kΩ
7
2008-05-22