In voltage regulation applications like a battery-powered system, where input voltage can fluctuate or drop to a voltage lower than the required output voltage, a buck converter can provide a well-regulated output voltage. To do this, it is critical to detect the change in the input voltage and quickly respond by either boosting up the low input voltage, or running in buck-only mode when the input is high enough for a regular buck conversion. A conventional 2-stage boost buck converter can implement this function, but it typically requires two controller ICs. This article discusses how using a single synchronous buck controller can output the most common voltage rails, and accommodate a wide range of operating input voltages from 3V to 40V, and at a reasonable BOM cost. Two 12W evaluation boards are used to demonstrate a 5V and 12V output.
Regulation over Wide Input Voltage Range
In systems where the input voltage varies or drops significantly, it is necessary to boost the voltage by turning on a boost converter to maintain the output regulation. However, the boost operation must be disabled when the input voltage is adequate or goes back to normal. This is achievable with a single synchronous buck controller, such as the ISL85403, that integrates a high-side MOSFET, high-side and low-side drivers, and is configurable to control a boost-buck converter. The low-side driver can drive a boost stage preceding the nonsynchronous buck stage controlled by the same IC. When the system input voltage drops too low to maintain regulation with the second stage buck converter, the first stage boost converter boosts the input voltage to regulate the output voltage. This provides a reliable converter solution in applications that have a wide operating input voltage range.
Figure 1 shows a typical boost-buck converter application circuit. The EXT_BOOST pin connects to the system input (often a battery) through a resistor divider. It monitors the boost input voltage to turn on/off the boost stage. When the input is high enough for the buck stage to regulate the output voltage, the boost stage is disabled -- either at startup or after the input recovers from a decline to normal. The threshold voltage used to turn on the boost PWM (from low-side gate drive LGATE) needs to have enough margin to cover the boost inductor and diode voltage drop, as well as the buck’s maximum duty cycle and conduction drop. This keeps the buck converter in regulation before the boost kicks in to boost up the input voltage. The R1 and R2 values set the boost turn-on threshold and hysteresis. The AUX VCC pin monitors the boost output voltage in a similar way.
Boost Buck Converter Application
The steady state DC transfer functions for the boost-stage output voltage, VBOOST and buck-stage output or the overall output, VOUT, are:
where VINPUT is the overall, and/or boost input voltage.
The steady-stage boost output voltage is derived as follows:
VBOOST is connected to the VIN pin of the IC, which is internally connected to a bias LDO to generate VCC, therefore keeping the IC operating even when VINPUT drops to a very low level.
This boost-buck configuration only requires one controller IC and one external MOSFET. Therefore, compared with the conventional 2-stage boost buck converters, this configuration has lower BOM cost while supporting an equally wide input voltage range.
Design Considerations for Low Input Operation
The boost-buck configuration supports a 2-stage boost buck converter with an input voltage as low as 3V. However, there are a few issues to consider under this operating condition.
- Selecting the Boost MOSFET
When starting up at 3V, it is critical that the boost stage switching MOSFET can fully turn on. Therefore, before selecting the MOSFET, carefully examine its gate charge waveforms and output characteristic charts. It is best to choose a MOSFET with a gate charge chart showing the plateau lower than 3V, although MOSFETs meeting this specification are limited. The Infineon BSZ025N04LS is a good choice. Not only can it switch on at 3V, it also has low ON resistance when fully turned on, which helps improve system efficiency in steady state operation.
- Setting the On/OFF Threshold for the Boost Stage
Another issue to consider is depending on the output voltage, the simple resistor divider on the EXT_BOOST pin may not be adequate to guarantee the boost converter’s proper turn-on and turn-off.
When powering up at a low input voltage, the voltage into the EXT_BOOST pin must be greater than 200mV for the boost converter to start up. When the input is high enough and the voltage into EXT_BOOST is greater than 800mV, the boost converter is disabled and only the buck stage operates.
If the boost converter needs to start at an input voltage of 3V, the lowest voltage the EXT_BOOST pin needs to see is 200mV. Then, an 800mV voltage (4x of 200mV threshold) into this pin would mean a 12V input voltage (4x of 3V), which means only the buck stage can switch at 12V. However, the regulation cannot be maintained if the targeted output voltage is 12V, or 9V-10V when considering all the overhead. Therefore, a resistor divider will not suffice for these specifications.
Figure 2 shows a component network that can be adjusted to ensure proper thresholds to turn on or turn off the boost converter. It replaces the resistor divider R1 and R2 in Figure 1.
Component Network to Adjust Threshold Voltages for EXT_BOOST
At power up from the battery, or system input, current flows through the resistor divider and forward biases the diode, which sets the voltage on the EXT_BOOST pin. Choose values for R1, R2 and R3 to guarantee a greater than 200mV voltage on EXT_BOOST at the low end of the input voltage range. R4 is a large value resistor, which pulls minimum current when input voltage is low. As the input goes up, R4 pulls more current and voltage on EXT_BOOST increases. Therefore, R4 can be adjusted to set the input voltage at which EXT_BOOST sees 800mV and the boost converter is disabled.
Note that the diode’s forward voltage changes over temperature, therefore, based on the diode’s characteristics, the resistor values need to yield proper threshold voltages over the application’s entire specified operating temperature range.