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3 / 10 page 3 TC1235 TC1236 TC1237 Inverting Dual (–VIN, –2VIN) Charge Pump Voltage Converters with Shutdown TC1235/6/7-1 3/24/00 © 2001 Microchip Technology Inc. DS21371A DETAILED DESCRIPTION The TC1235/1236/1237 dual charge pump converters perform both a –1x and –2x multiply of the voltage applied to the VIN pin. Output ‘– VIN’ provides a negative voltage inversion of the VIN supply, while output ‘-2 VIN’ provides a negative doubling inversion of VIN. Conversion is performed using two synchronous switching matrices and four exter- nal capacitors. When the shutdown input is held at a logic low both stages go into a very low power mode of operation consuming less than 1uA of supply current. Figure 1 (below) is a block diagram representation of the TC1235/1236/1237 architecture. The first switching stage inverts the voltage present at VIN and the second stage uses the ‘–VIN’ output generated from the first stage to produce the ‘–2VIN’ output function from the second stage switching matrix. Each device contains an on-board oscillator that syn- chronously controls the operation of the charge pump switch- ing matrices. The TC1235 synchronously switches at 12KHz, the TC1236 synchronously switches at 35KHz, and the TC1237 synchronously switches at 125KHz. The different oscillator frequencies for this device family allow the user to trade-off capacitor size versus supply current. Faster oscil- lators can use smaller external capacitors but will consume more supply current (see Electrical Characteristics Table). When the shutdown input is in a low state, the oscillator and both switch matrices are powered off placing the TC1235/ 1236/1237 in the shutdown mode. When the VIN supply input is powered from an external battery, the shutdown mode minimizes power consumption, which in turn will extend the life of the battery. Figure 1. Functional Block Diagram APPLICATIONS INFORMATION Output Voltage Considerations The TC1235/1236/1237 performs voltage conversions but does not provide any type of regulation. The two output voltage stages will droop in a linear manner with respect to their respective load currents. The value of the equivalent output resistance of the ‘-VIN’ output is approximately 50Ω nominal at +25 °C and VIN = +5V. The value of the ‘-2VIN’ output and is approximately 140 Ω nominal at +25°C and VIN = +5V. In this particular case, ‘-VIN’ is approximately – 5V and ‘–2VIN’ is approximately –10V at very light loads, and each stage will droop according to the equation below: VDROOP = IOUT x ROUT [-VIN OUTPUT] = VOUT1 = – (VIN – VDROOP1) [-2VIN OUTPUT] = VOUT2 = VOUT1 – (VIN – VDROOP2) where VDROOP1 is the output voltage droop contributed from stage 1 loading , and VDROOP2 is the output voltage droop from stage 2 loading. Charge Pump Efficiency The overall power efficiency of the two charge pump stages is affected by four factors: (1) Losses from power consumed by the internal oscil- lator, switch drive, etc. (which vary with input voltage, temperature and oscillator frequency). (2) I 2R losses due to the on-resistance of the MOSFET switches on-board each charge pump. (3) Charge pump capacitor losses due to effective series resistance (ESR). (4) Losses that occur during charge transfer (from the commutation capacitor to the output capacitor) when a voltage difference between the two capacitors exists. Most of the conversion losses are due to factor (2), (3) and (4) above. The losses for the first stage are given by Equation 1a and the losses for the second stage are given by Equation 1b. P1LOSS (2, 3, 4) = IOUT1 2 x R OUT1 where ROUT1 = [ 1 / [ fOSC(C1) ] + 8RSWITCH1 + 4ESRC1 + ESRCOUT1 ] —2VIN —VIN + + COUT1 COUT2 VIN SWITCH MATRIX (1st STAGE) SWITCH MATRIX (2nd STAGE) OSCILLATOR SHDN ENABLE ENABLE ENABLE C1 + C2 + |
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