Datasheet

744 053 221
C
IN
5.0 V
R
UVLO1
R
UVLO2
0.01 µF
255 kΩ
45.3 kΩ
R
LS
C
BOOT
C
POLE
8 V to 40 V
R
COMP
10 kΩ
2.2 µF
R
HS
35.7 kΩ
C
O
22 µF
C
COMP
220 Hµ
52.3 kΩ
7
6
5
8
L
O
C
O
220 pF
2
3
4
1
U1
0.33 µF
21
TPS54061
R
T
1240 kΩ
TPS54061
www.ti.com SLVSBB7C –MAY 2012–REVISED JANUARY 2014
DESIGN GUIDE – STEP-BY-STEP PROCEDURE Number 2
Figure 38. DCM Application Schematic
It is most desirable to have a power supply that is efficient and has a fixed switching frequency at low output
currents. A fixed frequency power supply will have a predictable output voltage ripple and noise. Using a
traditional continuous conduction mode (CCM) design method to calculate the output inductor will yield a large
inductance for a low output current supply. Using a CCM inductor will result in a large sized supply or will affect
efficiency from the large dc resistance an alternative is to operate in discontinuous conduction mode (DCM). Use
the procedure below to calculate the components values for designing a power supply operating in discontinuous
conduction mode. The advantage of operating a power supply in DCM for low output current is the fixed
switching frequency, lower output inductance, and lower dc resistance on the inductor. Use the frequency shift
and skip equations to estimate the maximum switching frequency.
For Designing an Efficient, Low Output Current Power Supply at a Fixed Switching Frequency
This example details the design of a low output current, fixed switching regulator design using ceramic output
capacitors. 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 5.0 V
Transient Response 37.5 to 75 mA load step ΔV
OUT
= 4%
Maximum Output Current 75 mA
Minimum Output Currert 1 mA
Input Voltage 24 V nom. 8 V to 40 V
Output Voltage Ripple 1 % of V
OUT
Switching Frequency 50 kHz
Start Input Voltage (rising VIN) 8 V
Stop Input Voltage (falling VIN) 6.8 V
The TPS54061 is designed for applications which require a fixed operating frequency and low output voltage
ripple at low output currents, thus, the TPS54061 does not have a pulse skip mode at light loads. Since the
device has a minimum controllable on time, there is an output current at which the power supply will pulse skip.
To ensure that the supply does not pulse skip at output current of the application the inductor value will be need
to be selected greater than a minimum value. The minimum inductance needed to maintain a fixed switching
frequency at the minimum load is calculated to be 227 µH using Equation 27. Since the equation is ideal and
was derived without losses, assume the minimum controllable light load on time, t
onminll
, is 180 ns. To maintain
DCM operation the inductor value and output current need to stay below a maximum value. The maximum
inductance is calculated to be 250 µH using Equation 28. A 744053221 inductor from Würth Elektronik is
selected. If CCM operation is necessary, use the previous design procedure.
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