Datasheet MAX889 (Maxim) - 5

High-Frequency, Regulated, 200mA, Inverting Charge Pump MAX889 Pin Description PIN NAME FUNCTION
1 IN Power-Supply Positive Voltage Input 2 CAP+ Positive Terminal of Flying Capacitor 3 GND Power Ground 4 CAP- Negative Terminal of Flying Capacitor 5 OUT Inverting Charge-Pump Output Shutdown Control Input. Drive SHDN low to shut down the MAX889. Connect SHDN to IN for 6 SHDN normal operation. Feedback Input. Connect FB to a resistor-divider from IN (or other positive reference voltage 7 FB source) to OUT for regulated output voltages. Connect to IN for free-run mode. 8 AGND Analog Ground
Detailed Description
the device switches continuously, the regulation scheme minimizes output ripple, and the output noise The MAX889 high-current regulated charge-pump DC- spectrum contains well-defined frequency components. DC inverter provides up to 200mA. It features the high- Feedback voltage is sensed with a resistor-divider est available output current while using small between an externally supplied positive reference or capacitors (Table 1). The three versions available differ the supply voltage and the negative inverted output. in their switching frequencies (fOSC)—MAX889R/ The feedback loop servos FB to GND. The effective MAX889S/MAX889T with fOSC = 500kHz/1MHz/2MHz, output impedance in regulation is 0.05Ω. The output respectively. Higher frequencies allow the use of small- remains in regulation until dropout is reached. Dropout er components (Table 1). Even smaller capacitor values depends on the output voltage setting and load current than those listed in Table 1 are suitable when the (see Output Voltage vs. Load Current in Typical devices are loaded at less than their rated output cur- Operating Characteristics). rent. Designed specifically for compact applications, a complete regulating circuit requires only three small
Free-Run Mode
capacitors and two resistors, Figure 1. In addition, the
(Unregulated Voltage Inverter)
MAX889 includes soft-start, shutdown control, short-cir- The MAX889 may be used in an unregulated voltage cuit, and thermal protection. inverter mode that does not require external feedback The oscillator, control circuitry, and four power MOSFET resistors, minimizing board space. Connecting FB to IN switches are included on-chip. The charge pump runs places the MAX889 in free-run mode. In this mode, the continuously at the operating frequency. During one-half charge pump operates to invert directly the input sup- of the oscillator period, switches S1 and S2 close ply voltage (VOUT = -(VIN - IOUT x RO)). Output resis- (Figure 2), charging the transfer capacitor (CFLY) to the tance is typically 2Ω and can be approximated by the input voltage (CAP- = GND, CAP+ = IN). During the following equation: other half cycle, switches S3 and S4 close (Figure 3), transferring the charge on CFLY to the output capacitor R (CAP+ = GND, CAP- = OUT). O ≅ [1 / (fOSC x CFLY) ] + 2RSW + 4ESRCFLY + ESRCOUT
Voltage Regulation
Voltage regulation is achieved by controlling the flying- capacitor charging rate. The MAX889 controls the The first term is the effective resistance of an ideal charge on CFLY by modulating the gate drive to S1 switched-capacitor circuit (Figures 2 and 3), and RSW (Figure 2) to supply the charge necessary to maintain is the sum of the charge pump’s internal switch resis- output regulation. When the output voltage droops, tances (typically 0.8Ω at VIN = 5V). The last two terms CFLY charges higher due to increased gate drive. Since take into consideration the equivalent series resistance
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