Datasheet

TPS54262-Q1
SLVS996C –SEPTEMBER 2009–REVISED JUNE 2010 www.ti.com
Example 2
For this example, we will start with the following known and target parameters:
Table 8.
minimum=8 V, maximum= 28 V,
Known Input voltage, VIN
typical=14 V
Output voltage, V
Reg
3.3 V ± 2%
Maximum output current, I
Load-Max
2 A
Ripple/ transient occurring in input voltage, ΔVIN 1% of VIN (minimum)
Reset threshold, VReg_RST 92% of V
Reg
Target
Overvoltage threshold, VReg_OV 106% of V
Reg
Undervoltage threshold, VReg_UV 95% of V
Reg
Transient response 0.25 A to 2 A load step, ΔV
Reg
5% of V
Reg
Power on Reset delay, PORdly 2.2 ms
Step 1. Calculate the Switching Frequency (f
sw
)
To reduce the size of output inductor and capacitor, higher switching frequency can be selected. It is important to
understand that higher switching frequency will result in higher switching losses, causing the device to heat up.
This may result in degraded thermal performance. To prevent this, proper PCB layout guidelines must be
followed (explained in the later section of this document).
Based upon the discussion in section Selecting the Switching Frequency, calculate the maximum and minimum
duty cycle.
Knowing V
Reg
and tolerance on V
Reg
, the V
Reg-Max
and V
Reg-Min
are calculated to be:
V
Reg-Max
= 102% of V
Reg
= 3.366 V and V
Reg-Min
= 98% of V
Reg
= 3.234 V.
Using Equation 6, the minimum duty cycle is calculated to be, D
Min
= 11.55%
Knowing t
ON-Min
= 150 ns from the device specifications, and using Equation 7, maximum switching frequency is
calculated to be, f
sw-Max
= 770 kHz.
Since the oscillator can also vary by ±10%, the switching frequency can be further reduced by 10% to add
margin. Also, to improve efficiency and reduce power losses due to switching, the switching frequency can be
further reduced by about 100 kHz. Therefore f
sw
= 593 kHz.
From Figure 23, R8 can be approximately determined to be, R8 = 170 kΩ.
Step 2. Calculate the Ripple Current (I
Ripple
)
Using Equation 40, for K
IND
= 0.2 (typical), inductor ripple current is calculated to be: I
Ripple
= 0.4 A.
The ripple current is chosen such that the converter enters discontinuous mode (DCM) at 20% of max load. The
20% is a typical value, it could go higher to a maximum of up to 40%.
Step 3. Calculate the Inductor Value (L1)
Using Equation 41, the inductor value is calculated to be, L
Min
= 12.3 µH. A closest standard inductor value can
be used.
Step 4. Calculate the Output Capacitor and ESR (C4, C12)
Calculate capacitance
To calculate the capacitance of the output capacitor, minimum load current must be first determined. Typically, in
standby mode the load current is 100 µA, however this really depends on the application. With this value of
minimum load current and using Equation 35, Equation 36, and Equation 37, C4 is calculated to be, C4 > 56 µF .
To allow wider operating conditions and improved performance in low-power mode, it is recommended to use a
100 µF capacitor. Higher value of output capacitor allows improved transient response during load stepping.
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