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MAX1522 Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX1522 Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 14 page Simple SOT23 Boost Controllers 6 _______________________________________________________________________________________ guarantees startup with input voltages down to 1.5V at VCC. The startup oscillator has a fixed 25% duty cycle and will toggle the MOSFET gate and begin boosting the output voltage. Once the output voltage exceeds the UVLO threshold, the normal control circuitry is used and the startup oscillator is disabled. However, N-chan- nel MOSFETs are rarely specified for guaranteed RDS(ON) with VGS below 2.5V; therefore, guaranteed startup down to 1.5V input will be limited by the MOS- FET specifications. Nevertheless, the MAX1524 boot- strapped circuit on the MAX1524 EV kit typically starts up with input voltage below 1V and no load. The MAX1522/MAX1523 may also be utilized by con- necting VCC to the output of an independent voltage regulator between 2.5V and 5.5V to allow operation with any combination of low or high input and output volt- ages. In this case, the independent regulator must sup- ply enough current to satisfy the IGATE current as calculated in the Power MOSFET Selection section when considering the maximum switching frequency as calculated in the CCM or DCM design procedure. On-Time SET Input The MAX1522/MAX1523/MAX1524 feature pin-selec- table fixed on-time control, allowing their operation to be optimized for various input/output voltage combina- tions. Connect SET to VCC for the 3µs fixed on-time. Connect SET to GND for the 0.5µs fixed on-time. The 3µs on-time setting (SET = VCC) permits higher than 80% guaranteed maximum duty factor, providing improved efficiency in applications with higher step-up ratios (such as 3.3V boosting to 12V). This setting is recommended for higher step-up ratio applications. The 0.5µs on-time setting (SET = GND) permits higher frequency operation, minimizing the size of the external inductor and capacitors. The maximum duty factor is limited to 45% guaranteed, making this setting suitable for lower step-up ratios such as 3.3V to 5V converters. Soft-Start The MAX1522/MAX1523/MAX1524 have a unique soft- start mode that reduces inductor current during startup, reducing battery, input capacitor, MOSFET, and induc- tor stresses. The soft-start period is fixed at 3.2ms and requires no external components. Fault Detection Once the soft-start period has expired, if the output voltage falls to, or is less than, 50% of its regulation value, a fault is detected. Under this condition, the MAX1522 disables the regulator until either SHDN is toggled low or power is removed and reapplied, after which it attempts to power up again in soft-start. For the MAX1523, the fault condition is not latched, and soft- start is repetitively reinitiated until a valid output voltage is realized. The MAX1524 has a latched fault detection, but when bootstrapped, the latch will be cleared when VCC falls below 2.37V. Shutdown Mode Drive SHDN to GND to place the MAX1522/MAX1523/ MAX1524 in shutdown mode. In shutdown, the internal reference and control circuitry turn off, EXT is driven to GND, the supply current is reduced to less than 1µA, and the output drops to one diode drop below the input voltage. Connect SHDN to VCC for normal operation. When exiting shutdown mode, the 3.2ms soft-start is always initiated. Undervoltage Lockout The MAX1522/MAX1523 have undervoltage lockout (UVLO) circuitry, which prevents circuit operation and MOSFET switching when VCC is less than the UVLO threshold (2.37V typ). The UVLO comparator has 70mV of hysteresis to eliminate chatter due to VCC input impedance. Applications Information Setting the Output Voltage The output voltage is set by connecting FB to a resis- tive voltage-divider between the output and GND (Figures 1 and 2). Select feedback resistor R2 in the 30k Ω to 100kΩ range. R1 is then given by: where VFB = 1.25V. Design Procedure Continuous vs. Discontinuous Conduction A switching regulator is operating in continuous con- duction mode (CCM) when the inductor current is not allowed to decay to zero. This is accomplished by selecting an inductor value large enough that the inductor ripple current becomes less than one half of the input current. The advantage of this mode is that peak current is lower, reducing I 2R losses and output ripple. In general, the best transient performance and most of the ripple reduction and efficiency increase of CCM are realized when the inductance is large enough to reduce the ripple current to 30% of the input current at maximum load. It is important to note that CCM circuits operate in discontinuous conduction mode (DCM) RR V V OUT FB 12 1 =− |
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