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LGH Amplifier Small Signal Model

A small signal model of the LGH amplifier is shown in

Figure 60.

The LGH amplifier consists of three distinct stages. The first is

a wide bandwidth integrator stage, followed by a fixed gain

level shift circuit. Finally, the level shift circuit drives a trans-

conductance (TCA) amplifier with an open collector sink

only output stage. Since the LGH output is internally

connected to the output of the voltage error amplifier, the

compensation components show up in the model and are

used by both stages, depending on which one is in use. Only

one stage should be in use at a time. When using LGH or if

the LGH input rises above the internal reference, the voltage

error amplifier acts as a 400uA current source pull up for the

EAO pin.

Figure 61 shows a small signal model of the modulator gain

when using the application circuit shown in Figure 59 with

two 3.4 V high current LED’s in series. RLED is the series

combination of the AC resistance of each LED, which is 0.2

Ohms. Rshunt is used to sense the current through the LED

string. It has a value of 0.050 Ohms in this case. The other

component values were defined earlier and remain the same

values. Equation (11) defines the transfer function of the

modulator and Equation (12) defines the pole of transfer

function. The transfer function of the LGH amplifier is

defined in Equation (13). The open loop gain of Eint is 2500

and ELS = 4.4.

Figure 60 — LGH Amplifier Small Signal Model

Vlgh

Rzi

Eint

Cint

ELS

GMlgh

400 u

leao

Chf

Rzi

Ccomp

Rout

Veao

Figure 61 — Lighting Application Modulator Gain Model

Rshunt

Vout

gMOD

rEQ

Cout

RLED

Veao

Vlgh

Fpled =

= 1.2 kHz

(12)

1

2 • π • ((RLED + Rshunt))//reQ) • C

OUT

Glgheao(s) = Eint (s) = • ELS • GMlgh •

(13)

Fphf =

= 580 kHz

(14)

Chf + Ccomp

2 • π • (Rzi // R

Gled(s) = gMOD • (rEQ • Rshunt)/((Rshunt + RLED + rEQ) + s (C

(11)

R

The integrator pole is determined by the external input resistor Rlgh and the internal Cint, which is 20 pF. Assuming Rlgh = 100 k

and Eint = 2500: