Regulated 3.3V/5.0V Step-Up/ Step-Down Charge PumpPin DescriptionPINNAMEFUNCTIONµMAXTQFN-EP Analog Power and Sense Input for Error Amplifier/Comparator. Connect to OUT at 1 12 AOUT output filter capacitor. MAX1595 2 1 SHDN Shutdown Input. When SHDN = low, the device turns off; when SHDN = high, the device activates. In shutdown, OUT is disconnected from IN. 3 2, 3 IN Input Supply. Can range from 1.8V to 5.5V. Bypass to GND with a 1μF capacitor. 4 4 GND Ground 5 5, 6 PGND Power Ground 6 7, 8 CXN Negative Terminal of the Charge-Pump Transfer Capacitor 7 9 CXP Positive Terminal of the Charge-Pump Transfer Capacitor 8 10, 11 OUT Output. Bypass to GND with output capacitor filter. Exposed Pad. Internally connected to GND. Connect to a large ground plane to — — EP maximize thermal performance. Not intended as an electrical connection point (TQFN package only). Detailed Description S2 IN OUT The MAX1595 charge pump provides either a 3.3V or 5V regulated output. It delivers a maximum 125mA load cur- rent. In addition to boost regulating from a lower supply, C S1 X it is also capable of buck regulating from supplies that exceed the regulated output by a diode drop or more. C Designed specifically for compact applications, a com- IN COUT plete regulator circuit requires only three small external capacitors. An innovative control scheme provides con- OSC stant frequency operation from medium to heavy loads, while smoothly transitioning to low-power mode at light loads to maintain optimum efficiency. In buck mode, switch S1 (Figure 1) is switched continuously to IN, while switch S2 alternates between IN and OUT. An amount of Figure 1. Unregulated Voltage Doubler charge proportional to the difference between the output voltage and the supply voltage is stored on CX, which than doubling the input voltage, the MAX1595 provides a gets transferred to the output when the regulation point is regulated output voltage of either 3.3V or 5.0V. reached. Maximum output ripple is proportional to the Shutdown difference between the supply voltage and the output voltage, as well as to the ratio of the transfer capacitor Driving SHDN low places the device in shutdown mode. (C The device draws 0.1μA of supply current in this mode. X) to the output capacitor (COUT). When driven high, the MAX1595 enters a soft-start The MAX1595 consists of an error amplifier, a 1.23V mode. Soft-start mode terminates when the output volt- bandgap reference, internal resistive feedback network, age regulates, or after 2ms, whichever comes first. In oscillator, high-current MOSFET switches, and shutdown shutdown, the output disconnects from the input. and control logic. Figure 1 shows an idealized unregulat- ed charge-pump voltage doubler. The oscillator runs at a Undervoltage Lockout 50% duty cycle. During one half of the period, the trans- The MAX1595 has an undervoltage-lockout that deacti- fer capacitor (CX) charges to the input voltage. During vates the devices when the input voltage falls below 1.6V. the other half, the doubler transfers the sum of CX and Below UVLO, hysteresis holds the device in shutdown until input voltage to the output filter capacitor (COUT). Rather the input voltage rises 40mV above the lockout threshold. 4_______________________________________________________________________________________
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