Datasheet
C
IN
L
O
8 V to 60 V
0.01 Fμ
100 Hμ
U1
TPS54061
3.3 V 200 mA
1
2
R
T
31.6 kΩ
C
POLE
36.5 kΩ
R
HS
2.2 Fμ
C
BOOT
R
UVLO2
33 pF
C
COMP
4700 pF
10 kΩ
R
LS
26.1 kΩ
R
COMP
R
UVLO1
143 kΩ
196 kΩ
C
O
10 Fμ
2
3
6
1
4
7
8
5
*
TPS54061
www.ti.com
SLVSBB7C –MAY 2012–REVISED JANUARY 2014
DESIGN GUIDE – STEP-BY-STEP DESIGN PROCEDURE No.1
* See Enable and Adjusting Undervoltage Lockout section
Figure 20. CCM Application Schematic
This example details the design of a continuous conduction mode (CCM) switching regulator design using
ceramic output capacitors. If a low output current design is see design procedure Number 2. A few parameters
must be known in order to start the design process. These parameters are typically determined at the system
level. For this example, we will start with the following known parameters:
Output Voltage 5.0V
Transient Response 50 to 150mA load step ΔV
OUT
= 4%
Maximum Output Current 200mA
Input Voltage 24 V nom. 8V to 60V
Output Voltage Ripple 0.5% of V
OUT
Start Input Voltage (rising VIN) 7.50V
Stop Input Voltage (falling VIN) 6.50V
Selecting the Switching Frequency
The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the
highest switching frequency possible since this will produce the smallest solution size. The high switching
frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that
switches at a lower frequency. The switching frequency is limited by the minimum on-time of the internal power
switch, the maximum input voltage, the output voltage and the frequency shift limitation.
Equation 6 and Equation 7 must be used to find the maximum switching frequency for the regulator, choose the
lower value of the two results. Switching frequencies higher than these values will result in pulse skipping or a
lack of overcurrent protection during short circuit conditions. The typical minimum on time, t
on
min, is 120ns for the
TPS54061. To ensure overcurrent runaway does not occur during short circuits in your design, use Equation 7 to
determine the maximum switching frequency. With a maximum input voltage of 60V, inductor resistance of 0.77
Ω, high side switch resistance of 3.0 Ω, low side switch resistance of 1.5Ω, a current limit value of 350 mA and a
short circuit output voltage of 0.1 V, the maximum switching frequency is 524 kHz and 1003 kHz in each case
respectively. A switching frequency of 400 kHz is used. To determine the timing resistance for a given switching
frequency, use Equation 5. The switching frequency is set by resistor R
T
shown in Figure 20. R
T
is calculated to
be 142 kΩ. A standard value of 143 kΩ is used.
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