TPS54162-Q1_15 Datasheet, PDF (31/42 Pages) Texas Instruments – 1-A 60-V STEP-DOWN DC/DC CONVERTER WITH LOW QUIESCENT CURRENT

TPS54162-Q1
www.ti.com
SLVSA32C – OCTOBER 2009 – REVISED MAY 2010
DESIGN EXAMPLE
The following example demonstrates 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.
For this example, we will start with the following known and target parameters:
Known
Target
Table 7.
Input voltage, VIN
Output voltage, V_Reg
Maximum output current, I_Load-Max
Ripple/ transient occurring in input voltage, ΔVIN
Reset threshold, VReg_RST
Overvoltage threshold, VReg_OV
Undervoltage threshold, VReg_UV
Transient response 0.25 A to 2 A load step, ΔV_Reg
Power on reset delay, PORdly
Minimum = 8 V, Maximum = 28 V, Typical = 14 V
3.3 V ± 2%
1A
1% of VIN (minimum)
92% of V_Reg
106% of V_Reg
95% of V_Reg
5% of V_Reg
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 = 3.324 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 193 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 = 0.2 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 = 29.1 µH. A closest standard inductor value can
be used.
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