Table 3. Design Parameters

8.2.1.2.1 Inductor Selection, Maximum Load Current

Because the PFM peak current control scheme is inherently stable, the inductor and capacitor value does not affect the stability of the regulator. The selection of the inductor together with the nominal load current, input, and output voltage of the application determines the switching frequency of the converter. Depending on the application, TI recommends inductor values between 2.2 to 47 μH. The maximum inductor value is determined by the maximum switch on-time of 6 μs (typical). The peak current limit of 375 mA (typical) must be reached within 6 μs for proper operation.

The inductor value determines the maximum switching frequency of the converter. Therefore, the user must select the inductor value for the maximum switching frequency at maximum load current of the converter and should not be exceeded. A good inductor value to start with is 4.7 μH. The maximum switching frequency is calculated as:

Equation 4.

where

• I_P = Peak current as described in Peak Current Control

Equation 5.

where

• L = Selected inductor value; TI recommends inductor values between 2.2 to 47 μH

If the selected inductor does not exceed the maximum switching frequency of the converter, as a next step, the switching frequency at the nominal load current is estimated as follows:

Equation 6.

where

• I_P = Peak current as described in Peak Current Control

Equation 7.

where

• L = Selected inductor value
• I_(LOAD) = Nominal load current
• V_F = Rectifier diode forward voltage (typically 0.3 V)

The smaller the inductor value, the higher the switching frequency of the converter, but the lower the efficiency.

The maximum load current of the converter is determined at the operation point where the converter starts to enter continuous conduction mode. The converter must always operate in DCM to maintain regulation.

Two conditions exist for determining the maximum output current of the converter. One condition is when the inductor current fall time is <400 ns, and the other condition is when the inductor current fall time is >400 ns.

One way to calculate the maximum available load current for certain operation conditions is to estimate the expected converter efficiency at the maximum load current. Find this number in the efficiency graphs shown in Figure 1 and Figure 2. Then, the maximum load current can be estimated:

Inductor fall time:

Equation 8.

For t_f ≥ 400 ns:

Equation 9.

For t_f ≤ 400 ns:

Equation 10.

where

• L = Selected inductor value
• η = Expected converter efficiency (typically between 70% to 85%)
• I_P = Peak current as described in Peak Current Control

Equation 11.

Equation 10 contains the expected converter efficiency that allows calculating the expected maximum load current the converter can support. Find the efficiency in the efficiency graphs shown in Figure 1 and Figure 2, or 80% can be used as a good estimation.

The selected inductor must have a saturation current which meets the maximum peak current of the converter as calculated in Peak Current Control. Use the maximum value for I_Lim (450 mA) for this calculation.

Another important inductor parameter is the dc resistance. The lower the dc resistance, the higher the efficiency of the converter. See Table 4 and Inductor Selection, Maximum Load Current.

8.2.1.2.2 Setting the Output Voltage

See Simplified Schematic. When the converter is programmed to the minimum output voltage, the DAC output (DO) equals the reference voltage of 1.233 V (typical). Therefore, only the feedback resistor network (R1) and (R2) determines the output voltage under these conditions. This gives the minimum output voltage possible and can be calculated as:

Equation 12.

The maximum output voltage is determined as the DAC output (DO) is set to 0 V:

Equation 13.

The output voltage can be digitally programmed by pulling the CTRL pin low for a certain period of time as described in Digital Interface (CTRL). Pulling the signal applied to the CTRL pin low and high again (for related timing see Electrical Characteristics) increases or decreases the DAC output DO (pin 3) one step where one step is typically 19.6 mV. A voltage step on DO of 19.6 mV (typical) changes the output voltage by one step and is calculated as:

Equation 14.

The possible output voltage range is determined by selecting R1, R2, and R3. A possible larger output voltage range gives a larger output voltage step size. The smaller the possible output voltage range, the smaller the output voltage step size.

To reduce the overall operating quiescent current in battery-powered applications a high-impedance voltage divider must be used with a typical value for R2 of ≤200 kΩ and a maximum value for R1 of 2.2 MΩ.

Some applications may not need the digital interface to program the output voltage. In this case, the output DO can be left open as shown in Figure 12, and the output voltage is calculated as for any standard boost converter:

Equation 15.

In such a configuration, a high-impedance voltage divider must also be used to minimize ground current, and TI recommends a typical value for R2 of ≤200 kΩ and a maximum value for R1 of 2.2 MΩ.

A feed-forward capacitor (C_(FF)), across the upper feedback resistor (R1), is required to provide sufficient overdrive for the error comparator. Without a feed-forward capacitor or with a too-small feed-forward capacitor value, the device shows double pulses or a pulse burst instead of single pulses at the switch node (SW). This can cause higher output voltage ripple. If a higher output voltage ripple is acceptable, the feed-forward capacitor can be left out also.

The lower the switching frequency of the converter, the larger the feed-forward capacitor value needs to be. A good starting point is to use a 10-pF feed-forward capacitor. As a first estimation, the required value for the feed-forward capacitor can be calculated at the operation point:

Equation 16.

where

• R1 = Upper resistor of voltage divider
• ƒ_S = Switching frequency of the converter at the nominal load current. (For the calculation of the switching frequency, see Inductor Selection, Maximum Load Current.)

For C_(FF), choose a value which comes closest to the calculation result.

The larger the feed-forward capacitor, the worse the line regulation of the device. Therefore, select the feed-forward capacitor as small as possible if good line regulation is of concern.

8.2.1.2.3 Output Capacitor Selection

For better output voltage filtering, TI recommends a low-ESR output capacitor. Ceramic capacitors have low-ESR values, but depending on the application, tantalum capacitors can also be used. For the selection of the output capacitor, see Table 5.

Assuming the converter does not show double pulses or pulse bursts on the switch node (SW), the output voltage ripple is calculated as:

Equation 17.

where

• I_P = Peak current as described in Peak Current Control

Equation 18.

where

• L = Selected inductor value
• I_O(LOAD)= Nominal load current
• ƒ_S(ILoad) = Switching frequency at the nominal load current as calculated previously.
• V_F = Rectifier diode forward voltage (typically 0.3 V)
• C_O = Selected output capacitor
• ESR = Output capacitor ESR value

8.2.1.2.4 Input Capacitor Selection

The input capacitor (C1) filters the high-frequency noise to the control circuit and must be directly connected to the input pin (VIN) of the device. The capacitor (C2) connected to the L pin of the device is the input capacitor for the power stage.

The main purpose of the capacitor (C2), that is connected directly to the L pin, is to smooth the inductor current. A larger capacitor reduces the inductor ripple current present at the L pin. The smaller the ripple current at the L pin, the higher the efficiency of the converter. If a sufficiently large capacitor is used, the input switch must carry only the dc current, filtered by the capacitor (C2), and not the high switching currents of the converter. A 4.7- or 10-μF ceramic capacitor (C2) is sufficient for most applications. For better filtering, this value can be increased without limit. See Table 5 for input capacitor recommendations.

8.2.1.2.5 Diode Selection

To achieve high efficiency, use a Schottky diode. The current rating of the diode must meet the peak current rating of the converter as it is calculated in Peak Current Control. Use the maximum value for I_(LIM) (450 mA) for this calculation. For the selection of the Schottky diode, see Table 6.

Table 7. Design Parameters