Datasheet

TPS54062

SLVSAV1B –MAY 2011–REVISED AUGUST 2012

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DESIGN GUIDE – STEP-BY-STEP PROCEDURE Number 2

Figure 36. DCM Application Schematic

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 3.3 V

Transient Response 0 to 15 mA load step ΔV

OUT

= 4%

Maximum Output Current 10 mA

Minimum Output Current 3 mA

Input Voltage 24 V nom. 10 V to 40 V

Output Voltage Ripple 0.5% of V

OUT

Switching Frequency 100 kHz

Start Input Voltage (rising VIN) 9 V

Stop Input Voltage (falling VIN) 8 V

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.

The TPS54062 is designed for applications which require a fixed operating frequency and low output voltage

ripple at low output currents, thus, the TPS54062 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 0.9mH using Equation 26. Since the equation is ideal and was

derived without losses, assume the minimum controllable light load on time, tonminll, is 350ns. To maintain DCM

operation the inductor value and output current need to stay below a maximum value. The maximum inductance

is calculated to be 1.42mH using Equation 27. A 744062102 inductor from Wurth Elektronik is selected. If CCM

operation is necessary, use the previous design procedure.

Product Folder Link(s): TPS54062