Datasheet

TPS54262-Q1
www.ti.com SLVS996C –SEPTEMBER 2009–REVISED JUNE 2010
DESIGN EXAMPLE
The following examples demonstrate the design of a high frequency switching regulator using ceramic output
capacitors. A few parameters must be known to start the design process. These parameters are typically
determined at the system level.
Example 1
For this example, we will start with the following known and target parameters:
Table 7.
Known Input voltage, VIN Minimum = 8 V, Maximum = 28 V, Typical = 14 V
Output voltage, V
Reg
5 V ± 2%
Maximum output current, I
Load-Max
1.8 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
= 5.1 V and V
Reg-Min
= 98% of V
Reg
= 4.9 V.
Using Equation 6, the minimum duty cycle is calculated to be, D
Min
= 17.5%
Knowing t
ON-Min
= 150 ns from the device specifications, and using Equation 7, maximum switching frequency is
calculated to be, f
sw-Max
= 1166 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 550 kHz. Therefore f
sw
= 500 kHz.
From Figure 23, R8 can be approximately determined to be, R8 = 205 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
= 0.36 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
= 22.8 µH. A closest standard inductor value can
be used.
Copyright © 2009–2010, Texas Instruments Incorporated Submit Documentation Feedback 31
Product Folder Link(s): TPS54262-Q1