TB7109F Datasheet, PDF (6/15 Pages) Toshiba Semiconductor – Power supply IC for LNB

Application Notes
TB7109F
DC-DC Converter section
Inductor Selection
The inductance required for inductor L can be calculated as follows:
L = ^{VIN1 − VOUT1} ⋅ ^VOUT1 ·············· (1)
f_OSC ⋅ΔI_L V_IN1
V_IN1: Input voltage (V)
V_OUT1: Output voltage (V)
f_OSC: Oscillation frequency = 400kHz (typ.)
ΔI_L: Inductor ripple current (A)
*: ΔI_L should be set to approximately 0.5A. The inductor should have a current rating greater than the peak
output current of 0.75A. If the inductor current rating is exceeded, the inductor becomes saturated,
leading to an unstable DC-DC converter operation.
When V_IN1 = 24V and V_OUT1 = 17V, the required inductance can be calculated as follows. Be sure to select an
appropriate inductor, taking the input voltage range into account.
L = ^{VIN1 − VOUT1} ⋅ ^VOUT1
f_OSC ⋅ΔI_L V_IN1
= ^{24V −17 V} ⋅ ^{17 V}
400kHz ⋅ 0.5A 24 V
= 24.8 μH
I_L
0
T= ¹
f_osc
T_ON
=
T
⋅
V_OUT1
V_IN1
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
2 based on the reference voltage of the error amplifier (1.215V typ.), which is connected to the Feedback pin,
V_FB. R_FB2 should be up to 10kΩ or so, because an extremely large-value R_FB2 incurs a delay due to parasitic
capacitance at the V_FB1 pin. It is recommended that resistors with a precision of ±1% or higher be used for
R_FB1 and R_FB2.
V_OUT1
=
V_FB1 ×
⎜⎜⎝⎛¹⁺
R_FB1
R_FB2
⎟⎟⎠⎞
= 1.215 V ×⎜⎜⎝⎛1+
R _FB1
R _FB2
⎟⎟⎠⎞
···· (2)
L_X
V_FB1
V_OUT1
Figure 3 Output Voltage Setting Resistors
Output Filter Capacitor Selection
Use a low-ESR electrolytic or ceramic capacitor as the output filter capacitor. Since a capacitor is generally
sensitive to temperature, choose one with excellent temperature characteristics. When the output voltage
exceeds 10V, the capacitance should be 4.7μF or greater for applications. The capacitance should be set
to an optimal value that meets the system’s ripple voltage requirement and transient load response
characteristics.
Rectifier Selection
A Schottky barrier diode should be externally connected to the TB7109F as a rectifier between the L_X and
GND pins. It is recommended CRS20I40B or equivalent be used as Schottky barrier diode. If a large voltage
overshoot is on the L_X pin, it reduces the voltage to connect a series CR network consisting of a resistor of R_S =
47Ω and a capacitor of C_S = 330pF with the Schottky barrier diode in parallel. Power loss of the Schottky
barrier diode tends to increase due to an increased reverse current caused by the rise in ambient temperature
and self-heating due to a supplied current. The rated current should therefore be derated to allow for such
conditions in selecting an appropriate diode.
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2011-05-19