Detailed Description

The MAX20404/MAX20405/MAX20406/MAX20406E are small, synchronous buck converters with integrated high-side and low-side switches. These devices are designed to deliver up to 6A with input voltages from +3V to +36V while using only 10μA quiescent current at no load (V_SUP = 12V, V_OUT = 3.3V). Voltage quality can be monitored by observing the PGOOD signal. The ICs can operate in dropout by running at 99% duty cycle, making them ideal for automotive and industrial applications.

The MAX20404/MAX20405/MAX20406/MAX20406E offer fixed output voltages (see the Ordering Information table) or an FB pin for an external resistor-divider. The frequency is internally fixed at 2.1MHz, 2.3MHz, 3MHz, or 400kHz, which allows for small external components and reduced output ripple, and guarantees no AM interference. The device automatically enters skip mode (SYNC pin pulled low) at light loads with an ultra-low-quiescent current of 10μA at no load. It can be ordered with spread-spectrum frequency modulation designed to minimize EMI-radiated emissions due to the modulation frequency. The architecture is an average current-mode control that allows much better noise rejection of the current loop. The IC comes with a small minimum ON time of 33ns to allow for large step-down ratios in single stage without skipping cycles.

The devices can also be used in a dual-phase configuration with the help of SYNCOUT and VEA. An innovative average current-mode control architecture provides noise immunity and accurate dynamic current sharing during transients. High-power designs with up to 12A of output current can be enabled with integrated switches using the IC's dual-phase capability. FC2QFN provides improved thermal and EMI performance. Symmetrical pinout across V_IN and PGND further improves the EMI performance, enabling low-noise designs.

Multiphase Operation

The MAX20404/MAX20405/MAX20406/MAX20406E are capable of dual-phase operation for high-current applications, and each IC can be configured as a controller or a target. The multiphase operation is intended for forced-PWM mode only. SYNCOUT will be 180 degrees out of phase with the controller clock. If the device is in Skip mode, then no clock will be present on SYNCOUT. To enable low-I_Q operation in dual-phase configuration, disable the EN of the target to turn OFF the IC and save quiescent current.

For a target, connect SYNCOUT to BIAS. When EN is high, there will be a procedure to detect if the IC is a controller (SYNCOUT not connected to BIAS) or a target. The VEA pin will be the voltage-error amplifier output for the controller or the current-error amplifier input for the target. Connect the VEA pin of the controller to the VEA pin of the target to ensure balanced current sharing between two phases. FB of the controller and target should be connected to their respective BIAS pins for internal fixed V_OUT setting. Since the target IC uses the outer voltage loop of the controller (through the VEA pin), the internal resistor-divider of the target IC is not used, and any mismatch in the output voltage is avoided. See Figure 1 for a dual-phase configuration setup using the MAX20404/MAX20405/MAX20406/MAX20406E.

For external resistor-divider configurations, use two separate resistor-dividers for each IC to avoid the FB pins of controller and target being connected together at the same point.

Figure 1. Typical Dual-Phase Configuration Setup Using Fixed Internal VOUT Setting — Figure 1. Typical Dual-Phase Configuration Setup Using Fixed Internal V_OUT Setting

Linear Regulator Output (BIAS)

The device includes a 1.8V linear regulator (V_BIAS) that provides power to the internal circuit blocks. Connect a 2.2µF ceramic capacitor from BIAS to GND. During startup, the bias regulator draws power from the input and switches over to the output after the startup is complete (if V_OUT > 2.5V). For output voltages less than 1.8V, the bias regulator is always tied to the input.

System Enable (EN)

An enable control input (EN) activates the devices from their low-power shutdown mode. EN is compatible with inputs from automotive battery level down to 3V. EN turns on the internal linear (BIAS) regulator. Once V_BIAS is above the internal lockout threshold (V_UVBIAS = 1.63V (typ)), the converter activates and the output voltage ramps up with the programmed soft-start time.

A logic-low at EN shuts down the device. During shutdown, the BIAS regulator and gate drivers turn off. Shutdown is the lowest power state and reduces the quiescent current to 4μA (typ). Drive EN high to bring the device out of shutdown.

Synchronization Input (SYNC)

The MAX20404/MAX20405/MAX20406/MAX20406E include a SYNC pin, which is a logic-level input used for operating-mode selection and frequency control. Connecting SYNC to BIAS or to an external clock enables forced fixed-frequency (FPWM) operation. Connecting SYNC to GND enables automatic skip-mode operation for better light-load efficiency. The ICs synchronize to an external clock at the rising edge applied at the SYNC pin. The devices synchronize to the external clock in two cycles. When the external clock signal at SYNC is absent for more than two clock cycles, the devices use the internal clock.

Soft-Start

The devices include a fixed, internal 2.5ms soft-start. Soft-start limits startup inrush current by forcing the output voltage to ramp up towards its regulation point.

Spread-Spectrum Option

The ICs feature enhanced EMI performance a with spread-spectrum option, which is available as a factory option. When spread spectrum is enabled, the operating frequency is varied at ±3% centered at the switching frequency. The modulation signal is a triangular wave with 4.5kHz frequency at 2.1MHz. Therefore, the switching frequency ramps down 3% and back to 2.1Mhz in 110μs, and also ramps up 3% and back to 2.1MHz in 110μs after which the cycle repeats. For operation at 400kHz, the modulation signal scales proportionally to 0.4/2.1. The internal spread spectrum is disabled if the devices are synchronized to an external clock. However, the devices do not filter the input clock in SYNC and pass any modulation (including spread spectrum) present on the the driving eternal clock.

Short-Circuit Protection

The devices feature a current limit that protects them against short-circuit and overload conditions at the output. In the event of a short-circuit or overload condition, the high-side MOSFET remains on until the inductor current reaches the specified LX current-limit threshold. The converter then turns the high-side MOSFET off and the low-side MOSFET on to allow the inductor current to ramp down. Once the inductor current crosses below the current-limit threshold, the converter turns on the high-side MOSFET again. This cycle repeats until the short or overload condition is removed.

A hard short is detected when the output voltage falls below 50% of the target (for fixed internal output) or 25% of the target (for external output) while in current limit. If this occurs, hiccup mode activates, and the output turns off for 25ms. The output then enters soft-start and powers back up. This repeats indefinitely while the short circuit is present. Hiccup mode is disabled during soft-start.