LM3421 datasheet(17/65 Pages) TI1

17 / 65 page

RSNS

ILED

RHSP

RHSN

HSN

HSP

High-Side

Sense Amplifier

CSH

1.24V

CCMP

RCSH

COMP

Error Amplifier

VSNS

To PWM

Comparator

LM3421/23

CFS

RFS

sets öP3

sets öP2

FREQUENCY (Hz)

100

-20

-40

-60

135

-45

-90

-135

-180

-225

1e-1

1e1

1e3

1e5

1e7

0°Phase Margin

öP1

PHASE

GAIN

öZ1

D x

LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

And the right half plane zero (

ωZ1) is:

(18)

Figure 22. Uncompensated Loop Gain Frequency Response

Figure 22 shows the uncompensated loop gain in a worst-case scenario when the RHP zero is below the output

pole. This occurs at high duty cycles when the regulator is trying to boost the output voltage significantly. The

RHP zero adds 20dB/decade of gain while loosing 45°/decade of phase which places the crossover frequency

(when the gain is zero dB) extremely high because the gain only starts falling again due to the high frequency

pole (not modeled or shown in figure). The phase will be below -180° at the crossover frequency which means

there is no phase margin (180° + phase at crossover frequency) causing system instability. Even if the output

pole is below the RHP zero, the phase will still reach -180° before the crossover frequency in most cases yielding

instability.

Figure 23. Compensation Circuitry

To mitigate this problem, a compensator should be designed to give adequate phase margin (above 45°) at the

crossover frequency. A simple compensator using a single capacitor at the COMP pin (CCMP) will add a dominant

pole to the system, which will ensure adequate phase margin if placed low enough. At high duty cycles (as

shown in Figure 22), the RHP zero places extreme limits on the achievable bandwidth with this type of

compensation. However, because an LED driver is essentially free of output transients (except catastrophic

failures open or short), the dominant pole approach, even with reduced bandwidth, is usually the best approach.

The dominant compensation pole (

ωP2) is determined by CCMP and the output resistance (RO) of the error

amplifier (typically 5 M

Ω):

Submit Documentation Feedback

Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1

LM3421 Datasheet(PDF) 17 Page - Texas Instruments

LM3421 Datasheet(HTML) 17 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Datasheet Download

Link URL