LMX2325TM datasheet(21/22 Pages) NSC

21 / 22 page

Application Information (Continued)

EXTERNAL CHARGE PUMP

The LMX PLLatinum series of frequency synthesizers are

equipped with an internal balanced charge pump as well as

outputs for driving an external charge pump Although the

superior performance of NSC’s on board charge pump elim-

inates the need for an external charge pump in most appli-

cations certain system requirements are more stringent In

these cases using an external charge pump allows the de-

signer to take direct control of such parameters as charge

pump voltage swing current magnitude TRI-STATE leak-

age and temperature compensation

One possible architecture for an external charge pump cur-

rent source is shown in

Figure 14 The signals wp and wr in

the diagram correspond to the phase detector outputs of

the 23152025 frequency synthesizers These logic sig-

nals are converted into current pulses using the circuitry

shown in

Figure 14 to enable either charging or discharging

of the loop filter components to control the output frequency

of the PLL

Referring to

Figure 14 the design goal is to generate a

5 mA current which is relatively constant to within 5V of the

power supply rail To accomplish this it is important to es-

tablish as large of a voltage drop across R5 R8 as possible

without saturating Q2

A voltage of approximately

300 mV provides a good compromise This allows the cur-

rent source reference being generated to be relatively re-

peatable in the absence of good Q1 Q2Q3 Q4 matching

(Matched transistor pairs is recommended) The wp and wr

outputs are rated for a maximum output load current of

1 mA while 5 mA current sources are desired The voltages

developed across R4 9 will consequently be approximately

258 mV or 42 mV less than R8 5 due to the current density

differences

0026 1n (5 mA1 mA)

through the Q1

Q2Q3 Q4 pairs

In order to calculate the value of R7 it is necessary to first

estimate the forward base to emitter voltage drop (Vfnp) of

the transistors used the VOL drop of wp and the VOH drop

of wr’s under 1 mA loads (wp’s VOL k 01V and (wrs VOH

01V)

Knowing these parameters along with the desired current

allow us to design a simple external charge pump Separat-

ing the pump up and pump down circuits facilitates the no-

dal analysis and give the following equations

R4 e

VR5 b VT 1n

isource

ip max

isource

R9 e

VR8 b VT 1n

isink

in max

isink

R5 e

VR5 (bp a 1)

ip max (bp a 1) b isource

R8 e

VR8 (bn a 1)

ir max (bn a 1) isink

R6 e

(Vp b VVOLwp) b (VR5 a Vfp)

ip max

R7 e

(VP b VVOHwp) b (VR8 a Vfn)

imax

EXAMPLE

Typical Device Parameters

bn e100 bp e 50

Typical System Parameters

VP e 50V

Vcntl e 05V–45V

Vwp e 00V Vwr e 50V

Design Parameters

ISINK e ISOURCE e 50 mA

Vfn e Vfp e 08V

Irmax e Ipmax e 1mA

VR8 e VR5 e 03V

VOLwp e VOHwp e 100 mV

TLW12339 – 39

FIGURE 14

Therefore select

R4 e R9 e

03V b 0026

1n(50 mA10 mA)

5mA

516X

R5 e

03V

(50 a 1)

10 mA

(50 a 1) b 50 mA

332X

R8 e

03V

(100 a 1)

10 mA

(100 a 1) b 50 mA

3156X

R6 e R7 e

(5V b 01V) b (03V a 08V)

10 mA

38 kX

http

www nationalcom

LMX2325TM Datasheet(PDF) 21 Page - National Semiconductor (TI)

LMX2325TM Datasheet(HTML) 21 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL