AN812. Paralleling the TC1121 to Reduce Output Resistance for Driving Higher Load Currents INTRODUCTION SUMMARY SINGLE TC1121 APPLICATION CIRCUIT

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1 Paralleling the TC1121 to Reduce Output Resistance for Driving Higher Load Currents Author: INTRODUCTION Patrick Maresca, Microchip Technology, Inc. Microchip Technology's TC1121 is a charge pump voltage converter specified to have a 100mA output current capability. The TC1121 has the benefits of selecting different internal charge pump switching frequencies (10KHz or 200KHz), driving the charge pump from an external clock, and a low power shutdown mode. An even greater benefit in certain applications where higher load currents are required (such as disk drive pre-amplifiers) is the ability to connect multiple TC1121s in parallel to drive loads greater than 100mA. The data in this application note compares measurements taken on a single TC1121 and two TC1121s connected in parallel. All measurements were made at ambient temperature (T A = 25 C). SINGLE TC1121 APPLICATION CIRCUIT Figure 1 shows the circuit configuration for measuring various load currents of a single TC1121. Two external capacitors (flying capacitor C1 and output capacitor C2) and a resistive load (comprised of RL1 and RL2) are required to measure the DC output voltage/droop and AC output voltage ripple under varying load conditions. To measure certain higher current loads, resistor RL1 was shorted. All measurements were made using an input voltage of 5.0V. Table 1 contains typical data for a constant load current of 100mA for different capacitor values and different charge pump oscillator frequencies. External capacitor values varying from 1µF to 47µF were used, and in each case two different charge pump oscillator frequencies were used: 1) the internal 200KHz, and 2) an external pump frequency that maximized the absolute value of the output voltage. Notice that the larger the external capacitors, the smaller the output voltage droop is for a constant 100mA load current. Table 2 contains typical data for varying load currents (from 25mA to 125mA) with varying external capacitor values. All measurements were made using the internal 200KHz charge pump switching frequency. Note that output voltage droop increases with higher load currents and smaller external capacitors. tor C2) and a resistive load (comprised of RL1 and RL2) are required to measure the DC output voltage droop under varying load conditions. To measure certain higher current loads, resistor RL1 was shorted. As before, all measurements were made using an input voltage of 5.0V. Table 3 contains typical data for varying load currents (from 25mA to 200mA) with varying external capacitor values. All measurements were made using the internal 200KHz charge pump switching frequency. As in the case of the single TC1121, the output voltage droop increases with higher load currents and smaller external capacitors, but the droop is significantly less than the single TC1121 configuration. The penalty the user pays is the expense of the additional TC1121 and one additional capacitor to achieve these higher load currents and to reduce the output voltage droop. SUMMARY Multiple TC1121 charge pumps can be paralleled to reduce output resistance and, therefore, increase the output current capability to load currents greater than 100mA. Voltage droop decreases with multiple TC1121s in parallel at the expense of additional component count and cost. 5V C1 2 CAP 4 CAP 8 TC1121 GND 3 SHDN FC OSC V OUT To External Func. Gen. V C R L1 R L2 PARALLEL TC1121 APPLICATION CIRCUIT Figure 2 shows the circuit configuration for measuring various load currents when two TC1121s are connected in parallel. Three external capacitors (flying capacitors C1A/C1B and output capaci- Notes: 1. When pin 7 open, internal 200KHz oscillator used. 2. Adjustable output load current (per 100Ω potentiomenter). 3. Shutdown mode disabled in this configuration Ω resistor used for 25mA load current measurement only. FIGURE 1: Single TC1121 application circuit. DS00812A-page 1

2 5V CAP SHDN 6 2 CAP SHDN 6 C1A 4 CAP TC1121(A) GND 3 FC OSC V OUT C1B To External Func. Gen. 4 CAP TC1121(B) GND 3 FC OSC V OUT To External Func. Gen. V C2 115 R L1 100 R L2 Notes: 1. Internal 200KHz oscillator used for all measurements. 2. Adjustable output load current (per 100Ω potentiometer). 3. Shutdown mode disabled in this configuration Ω resistor used for 25mA load current measurement only. FIGURE 2: Parallel TC1121s application circuit. V IN Flying Capacitor Output Capacitor Load V OUT V OUT Droop Osc. Osc. Freq. Output Ripple Voltage (V) C1 (µf) C2 (µf) Current (ma) Voltage (V) (V) (Int/Ext) (KHz) (mv p-p ) Internal External Internal External Internal External Internal External Internal External TABLE 1: Single TC1121 data summary with 100mA load current. DS00812A-page 2

3 V IN Flying Capacitor Output Capacitor Load Current V OUT Voltage V OUT Droop Osc. Osc. Freq. Output Ripple Voltage (V) C1 (µf) C2 (µf) (ma) (V) (V) (Int/Ext) (KHz) (mv p-p ) Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal Internal TABLE 2: Single TC1121 data summary at various load currents. DS00812A-page 3

4 V IN Flying Capacitors Output Capacitor Load Current V OUT Voltage V OUT Droop Improvement Osc. Osc. Freq. Voltage (V) C1A C1B (µf) C2 (µf) (ma) (V) (V) from Single (Int/Ext) (KHz) TC1121 (V) Internal Internal Internal Internal Internal N/A Internal N/A Internal N/A Internal Internal Internal Internal Internal Internal N/A Internal N/A Internal N/A Internal Internal Internal Internal Internal Internal N/A Internal N/A Internal N/A Internal Internal Internal Internal Internal Internal N/A Internal N/A Internal N/A Internal Internal Internal Internal Internal Internal N/A Internal N/A Internal N/A Internal 200 Note: Output ripple is similar to single TC1121 for identical values of output capacitor (C2) and load current. TABLE 3: Parallel TC1121s data summary at various load currents. DS00812A-page 4

5 Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, PIC, PICmicro, PICMASTER, PICSTART, PRO MATE, KEELOQ, SEEVAL, MPLAB and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. Total Endurance, ICSP, In-Circuit Serial Programming, Filter- Lab, MXDEV, microid, FlexROM, fuzzylab, MPASM, MPLINK, MPLIB, PICC, PICDEM, PICDEM.net, ICEPIC, Migratable Memory, FanSense, ECONOMONITOR, Select Mode and microport are trademarks of Microchip Technology Incorporated in the U.S.A. Serialized Quick Term Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. 2001, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July The Company s quality system processes and procedures are QS-9000 compliant for its PICmicro 8-bit MCUs, KEELOQ code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip s quality system for the design and manufacture of development systems is ISO 9001 certified Microchip Technology Inc. DS00812A-page 5

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