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TCM851 Datasheet(PDF) 5 Page - TelCom Semiconductor, Inc |
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TCM851 Datasheet(HTML) 5 Page - TelCom Semiconductor, Inc |
5 / 7 page 4-31 TELCOM SEMICONDUCTOR, INC. 7 6 5 4 3 1 2 8 REGULATED GaAs FET BIAS SUPPLY TCM850 TCM851 TCM852 TCM853 Shutdown Mode Shutdown inputs are provided to reduce TCM850/1/3 supply current during idle periods. When the shutdown input is in its active state, supply current over temperature is reduced to 0.5 µA (typ) TCM850/2/3 and 2µA (typ) TCM851. All shutdown inputs are TTL level compatible: TCM850 and TCM853 both have active low inputs (SHDN) while the TCM851 has an active high input. The TCM852 can be shutdown by setting the OSC input to a logic low level. The TCM852 exits the shutdown state when the clock signal is again restored. Charge Pump Frequency Control In applications where the charge pump switching fre- quency may cause interference or filtering problems, it is recommended the TCM852 be used. Switching frequency is determined by a 50kHz to 250kHz square wave signal applied to the OSC input. The signal applied to OSC can have a duty cycle between 40% and 60%. *Note: an external oscillator signal MUST be applied to the TCM852. The TCM852 does not have an on-board oscillator truebase. Capacitors Capacitors with low effective series resistance (ESR) should be used to maintain sufficient headroom across the linear post regulator. Recommended values for C1, C2, and C3 are 1 µF, 0.8Ω ESR. The recommended value for C4 is 10 µF, 0.2Ω. All capacitors should be either surface mount chip tantalum, or chip ceramic types. Board Layout The TCM850/1/2/3 typically finds use in applications where low output noise is important. To ensure good noise performance, please observe the following basic layout suggestions: (1) Mount all components in the circuit as close together as possible. (2) Keep trace lengths short; especially those to control inputs such as FB and CONT. This will reduce the effect of parasitic capacitance and inductance. (3) Minimize ground impedance by employing a ground plane. Noise and Ripple Measurement The TCM850/1/2/3 charge pump switching action causes small ground voltage differences between the TCM850/1/2 /3 circuit and the oscilloscope. These voltage differences cause ground currents in the probe wires inducing voltage spikes and result in erroneous readings. As a result, noise and ripple measurements should be made directly across output capacitor C4. Do not use the ground lead of the oscilloscope probe. Instead, remove the plastic cover from the probe tip and touch the probe ground ring directly to the negative side of C4. Tektronix chassis mount test jack (part number 131-0258) or a nail-type probe tip connect to the probe and minimize noise and ripple measurement error. Evaluation Kit TelCom Semiconductor offers evaluation kit (TCM850EV) for the TCM850-853. A second evaluation kit (TC7660EV) evaluates several TelCom charge pumps in- cluding the TC7660, TC7660S, TC7662B, TC962, TC682. This kit also supports the TCM850 (but not the TCM851- 853). |
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