AEC-Q100 Synchronous Buck Converter Sports Wide Input-Voltage Range
The synchronous buck converter is used to step a voltage down from a higher level to a lower level. On that front, Diodes Inc. recently revealed a further addition to its family of automotive-compliant dc-dc converters with the AP64060Q, a 600-mA device featuring an input-voltage range of 4.5 to 40 V.
Occupying nominal PCB space, the device fully integrates a 600-mΩ high-side power MOSFET and a 300-mΩ low-side power MOSFET to provide high-efficiency step-down dc-dc conversion (reaching 90%). The AP64060Q minimizes the external component count due to its adoption of peak current-mode control along with its integrated loop compensation network. It’s targeted for use in automotive powertrains, infotainment systems, and instrumentation clusters, as well as for vehicles’ exterior lighting.
The part’s low (typically 90 μA) quiescent current (Iq) and 1-μA shutdown current translates into enhanced light-load efficiency and enables systems to meet the <100-µA standby requirements of many automotive applications. Thanks to fast switching frequencies (2 MHz), it supports smaller accompanying inductors and capacitors. Its synchronous rectification eliminates the need for an external Schottky diode; peak current mode control and a built-in loop compensation network further reduce the overall component count.
The AP64060Q’s design is optimized for electromagnetic-interference (EMI) reduction. It has a proprietary gate-driver scheme to resist switching-node ringing without sacrificing MOSFET turn-on and turn-off times, which reduces high-frequency radiated EMI noise caused by MOSFET switching. The AP64060Q also features frequency spread spectrum (FSS) with a switching frequency jitter of ±6%, which reduces EMI by not allowing emitted energy to stay in any one frequency for a significant period of time.
Operating ambient temperature range is −40 to +125°C.
Protection Circuitry
The AP64060Q implements output overvoltage-protection (OVP) circuitry to minimize output-voltage overshoots during decreasing load transients. The high-side power MOSFET turns off and the low-side power MOSFET turns on when the output voltage exceeds its target value by 10% to prevent the output voltage from continuing to increase.
Cycle-by-cycle peak current-limit protection for the buck converter is provided by sensing the current through the internal high-side power MOSFET (Q1). While Q1 is on, the internal sensing circuitry monitors its conduction current. Once the current through Q1 exceeds the peak current limit, Q1 immediately turns off.
If the junction temperature of the device reaches the thermal shutdown limit of 170°C, the AP64060Q shuts down both its high- and low-side power MOSFETs. When the junction temperature is reduced to the required level (135°C typical), the device initiates a normal power-up cycle with soft-start.
An error amplifier generates the COMP voltage by comparing the voltage on the FB pin with an internal 0.8-V reference. An increase in load current causes the feedback voltage to drop. The error amplifier thus raises the COMP voltage until the average inductor current matches the increased load current. This feedback loop regulates the output voltage. The internal slope compensation circuitry prevents subharmonic oscillation when the duty cycle is greater than 50% for peak current-mode control.
Undervoltage lockout (UVLO) is implemented to prevent the IC from insufficient input voltages. The AP64060Q has a UVLO comparator that monitors the input voltage and the internal bandgap reference. The device disables if the input voltage falls below 3.8 V. In this UVLO event, both the high- and low-side power MOSFETs turn off.
The EN pin also can be used to program the UVLO thresholds.
PWM Operation
Regarding the fixed-frequency peak current-mode control, the internal 2-MHz clock’s rising edge initiates turning on the integrated high-side power MOSFET (Q1) for each cycle. When Q1 is on, the inductor current rises linearly and the device charges the output capacitor. The current across Q1 is sensed and converted to a voltage with a ratio of RT via the current-sense amplifier (CSA) block.
The CSA output is combined with an internal slope compensation (SE), resulting in VSUM. When VSUM rises higher than the COMP node, the device turns off Q1 and turns on the low-side power MOSFET (Q2). The inductor current decreases when Q2 is on. On the rising edge of next clock cycle, Q2 turns off and Q1 turns on. This sequence repeats every clock cycle.
The peak current-mode control, integrated loop compensation network, and built-in 4-ms soft-start time simplifies the part as well as minimizes the external component count.
As the load current approaches zero, the AP64060Q enters pulse-frequency-modulation (PFM) mode to increase the converter power efficiency at light-load conditions. When the inductor current decreases to 50 mA, zero cross detection circuitry on Q2 forces it off. The buck converter doesn’t sink current from the output when the output load is light and while the device is in PFM.
Because the AP64060Q works in PFM during light-load conditions, it can achieve power efficiency of up to 82% at a 5-mA load condition. The Iq of the AP64060Q is 90 μA typical under a no-load, non-switching condition.
When disabled, the device shutdown supply current is only 1 μA. If a voltage greater than the EN logic high threshold (typical 1.21V, rising) is applied, the AP64060Q enables all functions and the device initiates the soft-start phase. The EN pin is a high-voltage pin and can be directly connected to VIN to automatically start up the device as VIN increases.
A built-in 1-ms soft-start time prevents output-voltage overshoot and inrush current. When the EN voltage falls below its logic low threshold (typical 1.10 V, falling), the internal voltage discharges to ground and device operation is disabled.
The device, which comes in a TSOT26 package, costs $0.23 in 1,000-piece quantities. It is AEC-Q100 Grade 1 qualified, supports PPAP documentation (the PPAP package is a series of documents that need a formal certification/sign-off by the supplier and approval/sign-off by the customer) and is manufactured in IATF 16949 certified facilities (making the quality of services transparent and demonstrating process quality along the entire value chain).