REF LEVEL

0.000 dB

0.0 deg

100

1k

START 50.000 Hz

10k

STOP 50 000.000 Hz

/DIV

10.000 dB

45.000 deg

0

GAIN

PHASE

LM25005

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SNVS411C – JANUARY 2006 – REVISED MARCH 2013

R4, C5, C6

These components configure the error amplifier gain characteristics to accomplish a stable overall loop gain. One

advantage of current mode control is the ability to close the loop with only two feedback components, R4 and C5.

The overall loop gain is the product of the modulator gain and the error amplifier gain. The DC modulator gain of

the LM25005 is as follows:

(11)

(12)

For the design example of Typical Application Circuit and Block Diagram the following modulator gain vs.

frequency characteristic was measured as shown in Figure 16.

Figure 16. Gain and Phase of Modulator

Components R4 and C5 configure the error amplifier as a type II configuration which has a pole at DC and a

the crossover frequency of the loop gain. A single pole response at the crossover frequency yields a very stable

loop with 90 degrees of phase margin.

For the design example, a target loop bandwidth (crossover frequency) of 20 kHz was selected. The

frequency. This constrains the product of R4 and C5 for a desired compensation network zero 1 / (2

π R4 C5) to

be less than 2kHz. Increasing R4 while proportionally decreasing C5, increases the error amp gain. Conversely,

decreasing R4 while proportionally increasing C5, decreases the error amp gain. For the design example C5 was

selected for 0.01µF and R4 was selected for 49.9 k

Ω. These values configure the compensation network zero at

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