The MAX38650 is an ultra-low IQ (390nA) buck converter that steps-down from an input voltage range of 1.8V to 5.5V to a wide range of output voltages between 1.2V to 5V. The output voltage is either programmable (the MAX38650A) using a single external resistor or fixed from the factory (the MAX38650B). The external RSEL resistor on the RSEL pin programs the output voltage upon startup in the MAX38650A.
The buck converter automatically switches between low-power mode (LPM) and high-power mode (HPM) to better service the load, depending on the load current. The buck converter overregulates in LPM to allow the output capacitor to handle the transient load currents. The device supports 100% duty cycle operation.
The active discharge resistor in the MAX38650A/MAX38650B pulls OUT to ground when the part is in shutdown.
When VIN is above the UVLO rising threshold and the EN pin is pulled high (VEN > VIH), the MAX38650 is enabled. For the MAX38650A, there is a delay in reading the RSEL pin after which the soft-start mechanism begins.
When the EN pin is pulled low (VEN < VIL), the MAX38650 goes into disable (shutdown) mode. While in shutdown, 5nA of current is consumed from VIN. Reverse-current blocking from the output is active only when the part is disabled.
Select the RSEL resistor value by choosing the desired output voltage in Table 1.
The MAX38650A includes an RSEL pin to configure the output voltage and input UVLO threshold on startup. Resistors with a tolerance of 1% (or better) should be chosen, with nominal values specified in Table 1.
At startup, the MAX38650 sources up to 200μA during the select resistor detection time, typically for 600μs (tRSEL), to read the RSEL value.
Care must be taken that the total capacitance on this pin is less than 2pF. See the PCB Layout Guidelines for more information.
The RSEL output voltage selection method has many benefits:
- In conventional converters, current will be drawn from the output continuously through a feedback resistor-divider. In the MAX38650, 200μA of current will be drawn only during startup, which helps to increase efficiency at light loads.
- It provides lower cost and smaller size, since only one resistor is needed versus the two resistors in typical feedback connections.
- RSEL allows customers to stock just one part in their inventory system and use it in multiple projects with different output voltages just by changing a single standard 1% resistor.
- RSEL allows much higher internal feedback resistors instead of lower impedance external feedback resistors, thus enabling ultra-low power applications.
| TARGET OUTPUT VOLTAGE (V) | RSEL (kΩ) | INPUT UVLO THRESHOLD, RISING (V) |
|---|---|---|
| 2.5 | OPEN | 1.75 |
| 2 | 909 | 1.75 |
| 1.8 | 768 | 1.75 |
| 1.5 | 634 | 1.75 |
| 1.3 | 536 | 1.75 |
| 1.25 | 452 | 1.75 |
| 1.2 | 383 | 1.75 |
| 3.3 | 56.2 | 2.6 |
| 3 | 47.5 | 2.6 |
| 2.8 | 40.2 | 2.6 |
| 2.75 | 34 | 2.6 |
| 2.5 | 28 | 2.6 |
| 2 | 23.7 | 2.6 |
| 1.8 | 20 | 2.6 |
| 1.5 | 16.9 | 2.6 |
| 1.25 | 14 | 2.6 |
| 1.2 | 11.8 | 2.6 |
Note: The MAX38650B has an output voltage that is preprogrammed (no RSEL programming). Contact your Maxim Integrated representative to order a part with an output voltage preprogrammed in the output voltage range from 1.2V to 5.0V in 50mV steps. The input UVLO threshold for a preprogrammed device is 1.75V (VIN rising) with 50mV hysteresis.
The MAX38650 features 100% duty cycle operation. When the input voltage approaches the output voltage, the MAX38650 stops switching and enters 100% duty cycle operation. It connects the output to input through the high side power switch and the inductor. Entry into 100% duty cycle mode depends on the voltage ripple at the IN and OUT pins; refer to the Input and Output Capacitor Selection sections for recommended CIN and COUT capacitors. When the input voltage is increased again where it pulls VOUT to the level 5% above its target level, the converter restarts regulation. When the load is light, the device consumes only 1.05µA of current in 100% duty cycle mode while still protecting the inductor current from exceeding current limit.
The MAX38650 integrates a discharge resistor from the OUT pin to GND. This discharge resistor gets activated when converter is disabled, which helps discharge the output capacitor quickly. The typical value of the discharge resistance is 85Ω.