SC70 Inverting Charge Pumpswith ShutdownMAX1852/MAX1853Pin Description C1 PINNAMEFUNCTION 6 5 INPUT 1 OUT Inverting Charge-Pump Output C1+ C1- 2.5V TO 5.5V 4 1 NEGATIVE IN OUT OUTPUT 2 GND Ground C3 -1 ✕ VIN RL Shutdown Input. Drive this pin high MAX1852 C2 3 SHDN for normal operation; drive it low for MAX1853 ON 3 shutdown mode. SHDN OFF GND Power-Supply Voltage Input. Input 2 4 IN range is +2.5V to +5.5V. TE: ( Negative Terminal of the Flying 5 C1- Capacitor Figure 1. Typical Application Circuit Positive Terminal of the Flying 6 C1+ Capacitor resistances (typically 6Ω at VIN = +5V). The typical out- Detailed Description put impedance is more accurately determined from the The MAX1852/MAX1853 charge pumps invert the volt- Typical Operating Characteristics. age applied to their input. For highest performance use Shutdown low equivalent series resistance (ESR) capacitors (e.g., The MAX1852/MAX1853 have a logic-controlled shut- ceramic). down input. Driving SHDN low places the devices in a During the first half-cycle, switches S2 and S4 open, low-power shutdown mode. The charge-pump switch- switches S1 and S3 close, and capacitor C1 charges to ing halts, supply current is reduced to 2nA. the voltage at IN (Figure 2). During the second half- Driving SHDN high will restart the charge pump. The cycle, S1 and S3 open, S2 and S4 close, and C1 is level switching frequency and capacitor values determine how shifted downward by VIN volts. This connects C1 in par- soon the device will reach 90% of the input voltage. allel with the reservoir capacitor C2. If the voltage across C2 is smaller than the voltage across C1, charge flows Applications Information from C1 to C2 until the voltage across C2 reaches Capacitor Selection -VIN. The actual voltage at the output is more positive The charge-pump output resistance is a function of the than -VIN since switches S1–S4 have resistance and the ESR of C1 and C2. To maintain the lowest output resis- load drains charge from C2. tance, use capacitors with low ESR. (See Table 1 for a Efficiency Considerations list of recommended manufacturers.) Tables 2 and 3 The efficiency of the MAX1852/MAX1853 is dominated suggest capacitor values for minimizing output resis- by their quiescent supply current (I tance or capacitor size. Q) at low output cur- rent and by their output impedance (ROUT) at higher Flying Capacitor (C1) output current; it is given by: Increasing the flying capacitor’s value reduces the out- I I x R put resistance. Above a certain point, increasing C1’s OUT η ≅ 1 OUT OUT − capacitance has negligible effect because the output I +I V OUT Q IN resistance is then dominated by internal switch resis- tance and capacitor ESR. where the output impedance is roughly approximated by: Output Capacitor (C2) 1 Increasing the output capacitor’s value reduces the R ≅ 2R 4ESR ESR OUT SW C1 C2 output ripple voltage. Decreasing its ESR reduces both f x C1 ( ) + + + OSC output resistance and ripple. Lower capacitance values The first term is the effective resistance of an ideal can be used with light loads if higher output ripple can switched-capacitor circuit (Figures 3a and 3b), and be tolerated. Use the following equation to calculate the R peak-to-peak ripple: SW is the sum of the charge pump’s internal switch _______________________________________________________________________________________5
