The Functions of BOOT and Other Pins in Common DC-DC Chips

Common Pin Names and Functions in Power ICs: Understanding Special Pins in DC-DC Chips

In addition to the regular input and output pins, common DC-DC chips also have some special pins. What are the full names of these pins and what are their functions? To help everyone quickly identify chip functions, this article will introduce the common pin names in power ICs.

01 RT/CLK/SYNC

RT is the internal oscillator frequency control pin of the DC-DC, where R stands for resistor. Generally, a resistor is connected to the ground to control the working frequency of the DC-DC. The resistance value is usually provided in the form of a table in the datasheet.

SYNC and CLK are external clock input pins for the DC-DC. SYNC stands for synchronizing, and CLK stands for clock. These pins connect to an external clock source to control the operating frequency of the DC-DC. The advantage of external clock input is that when there are multiple DC-DCs, using the same clock can reduce the total input current ripple, which is beneficial for the EMC of the circuit.

RT and CLK may sometimes be the same pin.

02 PG/PERGD/PGOOD

The power status indicator pin, which is generally an open-drain output, requires a pull-up resistor. This pin can be connected to an LED indicator light or an MCU IO. It outputs a high level when the power output is normal and a low level when the power output is abnormal.

03 FB/VSENSE

The FB pin is the feedback pin. Once negative feedback is formed, it fixes the voltage. The voltage value is provided in the datasheet. For example, in the figure below, the FB voltage Vfb is known to be 0.6V, so the output voltage = Vfb * (1 + R1/R2).

04 COMP

This pin is the output of the error amplifier and the input of the PWM comparator. It is generally connected in series with a capacitor and resistor for frequency compensation. Depending on the principle, it may be connected to ground or the FB pin. Why is frequency compensation necessary? During the operation of the power IC, the load may suddenly increase or decrease, requiring high dynamic performance from the power IC to avoid voltage drops or overshoots. If the output voltage is unstable during load changes, the capacitance and resistance connected to the COMP pin should be adjusted.

05 BOOT/VBST

The BOOT pin is the boost capacitor pin for the high-side FET gate driver. This pin is generally connected to a 0.1uF capacitor to the SW pin to increase the gate drive voltage.

As shown in the figure, to turn on the high-side NMOS, the gate voltage G of the NMOS should be higher than the source voltage SW. However, when the SW voltage is close to the input voltage VIN, VG cannot be higher than the SW voltage. Therefore, the VBST capacitor is needed, using the characteristic that the capacitor voltage cannot突变, to increase the supply voltage of the HS operational amplifier to be greater than the SW voltage, i.e., greater than the input voltage VIN, so that the G voltage is greater than SW.

06 CS/CSG

C stands for Current, S stands for Sensor, and G stands for GND. Generally, a shunt resistor is connected between CS and CSG to read the current. ICs with adjustable current limiting protection usually have these two pins.

07 EN/ON#OFF

EN is short for Enable Pin, which means the enable pin. It requires a certain voltage or grounding to control the switch of the IC. In most applications, it is enabled by connecting a resistor to the input pin.

08 EP/PAD

EP stands for Exposed Thermal PAD, which is the exposed thermal pad of the package. It should be connected to ground to facilitate heat dissipation.

09 SW/PH

The SW pin is generally connected to the inductor because this pin is also connected to the MOSFET switch inside the DC-DC, so SW is short for Switch.

10 Reference ICs are as follows:

TI: LM5118, TPS54331, TPS54360B, TPS56320x, TPS54231, TLV62569, TPS51200, TLV62130x, TPS54240, etc.