Detailed Description
The MAX25231 family of small, current-mode-controlled buck converters features synchronous rectification and requires no external compensation network. The devices are designed for 1.2A and can stay in dropout by running at 99% duty cycle. They provide an accurate output voltage within the 5.5V to 18V input range. Voltage quality can be monitored by observing the PGOOD signal. The devices operate at a frequency of 2.1MHz (typ), which allows for small external components, reduced output ripple, and guarantees no AM band interference.
The devices feature an ultra-low 3.5μA (typ) quiescent supply current in standby mode. The devices center standby mode automatically at light loads if HSFET does not turn on for eight consecutive clock cycles. The devices operate from a 3.5V to 36V supply voltage and can tolerate transients up to 40V, making them ideal for automotive applications. The devices are available in factory-trimmed fixed output voltages of 5V and 3.3V and are programmable with an external resistor-divider (MAX25231ATCD only). For fixed output voltages outside of 3.3V and 5V, contact factory for availability.
Enable Input (EN)
The device is activated by driving EN high. EN is compatible from a 3.3V logic level to automotive battery levels. EN can be controlled by microcontrollers and automotive KEY or CAN inhibit signals. The EN input has no internal pullup/pulldown current to minimize the overall quiescent supply current. To realize a programmable undervoltage-lockout level, use a resistor-divider from SUP to EN to AGND.
BIAS/UVLO
The device features undervoltage lockout. When the device is enabled, an internal bias generator turns on. LX begins switching after VBIAS has exceeded the internal undervoltage-lockout level, VUVLO = 2.73V (typ).
Soft-Start
The device features an internal soft-start timer. The output voltage soft-start ramp time is 2.5ms (typ). If a short circuit or undervoltage is encountered after the soft-start timer has expired, the device is disabled for 6ms (typ) and then reattempts soft-start again. This pattern repeats until the short circuit has been removed.
Oscillator/Synchronization and Efficiency (SYNC)
The device has an on-chip oscillator that provides a 2.1MHz (typ) switching frequency. Depending on the condition of SYNC, two operation modes exist. If SYNC is unconnected or at AGND, the device operates in highly efficient pulse-skipping mode. If SYNC is at BIAS or has a clock applied to it, the device is in forced-PWM mode (FPWM). The device can be switched during operation between FPWM mode and skip mode by switching SYNC.
Skip-Mode Operation
Skip mode is entered when the SYNC pin is connected to ground or is unconnected and the peak load current is < 150mA (typ). In this mode, the high-side FET is turned on until the current in the inductor is ramped up to 150mA (typ) peak value and the internal feedback voltage is above the regulation voltage (1.0V, typ). At this point, both the high-side and low-side FETs are turned off. Depending on the choice of the output capacitor and the load current, the high-side FET turns on when OUT
(valley) drops below the 1.0V (typ) feedback voltage. For fixed output voltage parts, when the device is in skip mode, the internal high-voltage LDO is turned off after the startup is complete to reduce the input current. VBIAS is supplied by the output in this condition.
Achieving High Efficiency at Light Loads
The device operates with very low quiescent current at light loads to enhance efficiency and conserve battery life. When the device enters skip mode, the output current is monitored to adjust the quiescent current.
When the output current is less than approximately 5mA, the devices operate in the lowest-quiescent-current mode, also called standby mode. In this mode, the majority of the internal circuitry (excluding that necessary to maintain regulation) in the device, including the internal high-voltage LDO, is turned off to save current. Under no load and with skip mode enabled, the IC typically draws 4.5μA for the 3.3V parts, and 6.6μA for the 5.0V parts. For load currents greater than 5mA, the device enters normal skip mode, still maintaining very high efficiency.
Controlled EMI with Forced Fixed Frequency
In FPWM mode, the device attempts to operate at a constant switching frequency for all load currents. For tightest frequency control, apply the operating frequency to SYNC. The advantage of this mode is a constant switching frequency, which improves EMI performance;
the disadvantage is that considerable current can be wasted. If the load current during a switching cycle is less than the current flowing through the inductor, the excess current is diverted to AGND.
Extended Input Voltage Range
In some cases, the device is forced to deviate from its operating frequency, independent of the state of SYNC. For input voltages above 18V, the required duty cycle to regulate its output may be smaller than the minimum on- time (66ns, typ). In this event, the device is forced to lower its switching frequency by skipping pulses.
If the input voltage is reduced and the device approaches dropout, the device tries to turn on the high-side FET continuously. To maintain gate charge on the high-side FET, the BST capacitor must be periodically recharged. To ensure proper charge on the BST capacitor when in dropout, the high-side FET is turned off every 20μs and the low-side FET is turned on for about 200ns. This gives an effective duty cycle of > 99% and a switching frequency of 50kHz when in dropout.
Spread-Spectrum Option
The device has an optional spread spectrum enabled by the SPS pin. If SPS is pulled high, then the internal operating frequency varies by ±6% relative to the internally generated operating frequency of 2.1MHz (typ). Spread spectrum is offered to improve EMI performance of the device.
The internal spread spectrum does not interfere with the external clock applied on the SYNC pin. It is active only when the device is running with an internally generated switching frequency.
The device features an open-drain power-good output. PGOOD is an active-high output that pulls low when the output voltage is below 93% of its nominal value. PGOOD is high impedance when the output voltage is above 93.5% of its nominal value. Connect a 20kΩ (typ) pullup resistor to an external supply or the on-chip BIAS output.
Overcurrent Protection
The device limits the peak output current to 1.9A (typ). The accuracy of the current limit is ±12%, which makes selection of external components very easy. To protect against short-circuit events, the device shuts off when OUT is below 50% of OUT voltage and an overcurrent event is detected. The device attempts a soft-start restart every 7ms and remains off if the short circuit has not been removed. When the current limit is no longer present, it reaches the output voltage by following the normal soft-start sequence. If the device's die reaches the thermal limit of 175°C (typ) during the current-limit event, it immediately shuts off.
Thermal-Overload Protection
The device features thermal-overload protection, turning off when the junction temperature exceeds +175°C (typ). Once the device cools by 15°C (typ), it turns back on with a soft-start sequence.