TCM850 Просмотр технического описания (PDF) - TelCom Semiconductor Inc => Microchip

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TCM850

REGULATED GaAs FET BIAS SUPPLY

TelCom Semiconductor Inc => Microchip 

REGULATED GaAs FET BIAS SUPPLY

1 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 0.5µA (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 1µF, 0.8Ω ESR. The recommended value for C4 is

10µF, 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

2 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

3 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

4 (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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TELCOM SEMICONDUCTOR, INC.

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