Inverting Buck-Boost Application for the LM5168

1 Inverting Buck-Boost Application for LM5168

The LM5168 is a synchronous buck converter with a wide input voltage range from 6 V to 115 V and maximum output current of 300 mA. The LM5168 can be configured as an inverting buck-boost (IBB) converter with a negative output voltage. This application note demonstrates how the LM5168 can be used as an inverting buck-boost converter, along with optional design considerations for inverting buck-boost converters such as a PGOOD or EN level-shifter. If higher output current is required, the LM5168 is pin-to-pin compatible with the 600-mA rated LM5169. Automotive grade versions, the LM5168-Q1 and LM5169-Q1, are also available.

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1 Inverting Buck-Boost Topology

1.1 Concept

For the standard buck converter, the inductor is connected to V_OUT and the switch pin (SW) of the LM5168. In order to change to an inverting buck-boost topology, the V_OUT and ground nodes of the circuit must be reversed. With the nodes reversed, the LM5168 can now invert the output voltage from the input voltage.

To change an LM5168 buck converter to an inverting buck-boost, reassign the buck converter V_OUT to system ground, and the old buck system ground to -V_OUT. The input capacitor will need to be reconnected to the new system ground, and a new bypass capacitor, C_IO, is needed between V_IN and -V_OUT. The positive input and the feedback resistors will remain the same as in the buck converter. To adjust the output of the inverting buck-boost, calculate the feedback resistor values as if it was a buck converter. For further reading on the inverting buck-boost topology, refer to the Working with Inverting Buck-Boost Converters application note. The schematics in #ID-B6C9A22A-CA23-4644-FA3C-C4FA7281CD60 show the changes that have to be made when configuring the LM5168 buck converter as an inverting buck-boost converter.

Figure 1-1 Converting From Buck to Inverting Buck-Boost Topology

1.2 Output Current Calculations

Changing to an inverting buck-boost topology will have an impact on the maximum possible output current. For an inverting buck-boost converter, the inductor current will always be larger than the output current. As a result, the maximum possible output current is calculated by:

Equation 1.

$I_{O U T} (I B B) = I_{O U T_B u c k} \times (1 - D)$

where:

I_{OUT_Buck} is the maximum buck converter DC output current
D is the duty cycle

The duty cycle can be calculated as:

Equation 2.

$D = \frac{V o u t}{V o u t - V i n \times η}$

V_OUT is represented as the negative output voltage of the inverting buck-boost converter. The efficiency term in the duty cycle equation helps account for power losses to provide a more accurate calculation of the output current. For example, if the output voltage is -24 V, the input voltage is 12 V, and the efficiency is assumed to be 85%, then the duty cycle is:

Equation 3.

$D = \frac{- 24}{- 24 - 12 \times 0.85} = 0.702$

In the case of the LM5168, which has a maximum current of 300 mA, the resulting maximum output current of the inverting buck-boost converter would be:

Equation 4.

$I_{O U T} (I B B) = 300 m A \times (1 - 0.702) = 89 m A$

Table 1-1 Maximum Output Current Calculations for the LM5168

V_OUT (V)	V_IN (V)	IOUT_Buck (A)	η	D	I_OUT (A)
–12	24	0.3	0.85	0.370	0.189
–24	24	0.3	0.85	0.541	0.138
–36	24	0.3	0.85	0.638	0.109
–48	24	0.3	0.85	0.702	0.089

Figure 1-2 Maximum Output Current Calculations for the LM5168 with Assumed 85% Efficiency