Changes in peripheral components has negligible effect on the heat generated by S6BP501A/S6BP502A.
Several factors in a DC/DC converter lead to heat generation such as the following, mainly due to the inductor and switching FET. However, peripheral components do not significantly affect the heat generated by S6BP501A/S6BP502A.
- Conduction loss by the ON-resistance of the switching FET
- Switching loss at ON/OFF transition of the switching FET
- Conduction loss by the DC resistance (RDC) of the inductor
- Core loss by the magnetic material of the inductor
S6BP501A/S6BP502A consists of a primary channel (DD3V) that generates 3.3 V from the battery, and secondary channels (DD5V and DD1V) that generate 5 V and 1.2 V from the output of DD3V. The block diagram of S6BP501A/S6BP502A is shown below.
Figure1. S6BP501A/S6BP501A Block Diagram
The main heat factor of the DC/DC converter in S6BP501A/S6BP502A is the three DC/DC converters and the DD3V load switch.
The temperature of S6BP501A/S6BP502A rises by the heat generation factor built-in the S6BP501A/S6BP502A. Table 1 shows the heat generation factor built in the S6BP501A/S6BP502A.
Table 1. Relationship between S6BP501A/S6BP502A and heat generation components
The DD3V load switch also is built-in, but it is not included for the consideration because it is not affected by the DC/DC converter peripheral components.
First, regarding DD3V, which is the primary channel, the switching FET and inductor, which are the main factor of heat generation, are not built-in. Therefore, the heat of peripheral components increases if the DD3V conversion efficiency deteriorates with changes in peripheral components.
This means that the heat generated by S6BP501A/S6BP502A itself does not increase. Also, the S6BP501A/S6BP502A temperature does not rise even if the DD3V load current increases because the conversion efficiency of the secondary channels (DD1V, DD5V) has deteriorated.
Next, the heat generated by the secondary channels (DD1V, DD5V) themselves is generated by the built-in switching FET. If the efficiency changes because of a change in the inductor, the inductor's temperature increases. This causes the switching FET ON duty cycle to change. (The ON duty cycle of the high-side FET in DD1V and low-side FET in DD5V increase). As a result of this, the PMIC loss may increase. The ON resistance of each switching FET of the secondary channel (DD1V, DD5V) is as follows:
Table2. Switchin FET ON resistance
DD1V High-side FET
DD1V Low-side FET
DD5V High-side FET
DD5V Low-side FET
Consider the example of a 2-A load current condition with DD1V. When the inductor is changed from CLF6045NI-1R5N-D (RDC = 13 mΩ typ.) to TFM252012ALMA1R5M (RDC = 52 mΩ typ.), the inductor conduction loss caused by its DC resistance increases 4 times (see Note 2).
On the other hand, the ON duty cycle of the high-side FET that has a higher ON resistance and is built in the S6BP501A S6BP502A increases, the increase in switching FET loss is only 0.6% (see Note 2).
Therefore, the increase in power loss of S6BP501A/S6BP502 (PMIC) caused by the change in peripheral components is very small and hardly affects the heat generated by the PMIC.
Note 1: As a premise, heat conduction from components that generate heat to the PMIC is ignored.
Note 2: Inductor’s loss estimation
In the case of CLF6045NI-1R5N-D:
Conduction loss caused by the DC resistance of the inductor = 2A^2 × 0.052 Ω = 0.208 W
In the case of TFM252012ALMA1R5M:
Conduction loss caused by the DC resistance of the inductor =2A^2 × 0.013 Ω = 0.052 W
Therefore, when changing from CLF6045NI-1R5N-D to TFM252012ALMA1R5M, the inductor conduction loss rate is 0.208 W/0.052 W = 400% -> x4
Note 3: FET’s loss estimation
High-side FET ON duty cycle = (1.2 V + 2 A × 0.1 Ω + 2 A × 0.013 Ω) / (3.3 V – 2 A × 0.13 Ω + 2 A × 0.1 Ω) = 44.0%
Switching FET conduction loss = 2A^2 × 0.13 Ω × 44% + 2 A^2 × 0.1 Ω × 56% = 0.4528 W
High-side FET ON duty cycle = (1.2 V+2 A × 0.1 Ω + 2 A × 0.052 Ω) / (3.3 V – 2 A × 0.13 Ω + 2 A × 0.1 Ω) = 46.4%
Switching FET conduction loss = 2 A^2 × 0.13 Ω × 44% + 2 A^2 × 0.1 Ω × 56% = 0.45568 W
Therefore, when CLF6045NI-1R5N-D is changed to TFM252012ALMA1R5M, the switching FET conduction loss rate is 0.45568 W / 0.4528 W = 100.6% -> +0.6%
If the input voltage and output current are constant, even if peripheral components are changed, the switching loss of the FET does not change significantly.
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