LM5005MH National Semiconductor, LM5005MH Datasheet - Page 17

• Manufacturer Part Number: LM5005MH
• Description: IC, BUCK REGULATOR, TSSOP-20
• Manufacturer: National Semiconductor

Specifications of LM5005MH

• Primary Input Voltage: 75V
• No. Of Outputs: 1
• Output Voltage: 1.225V
• Output Current: 2.5A
• No. Of Pins: 20
• Operating Temperature Range: -40°C To +125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application Information
Components R4 and C5 configure the error amplifier as a
type II configuration which has a pole at unity and a zero at
f
modulator pole leaving a single pole response at the cross-
over frequency of the loop gain. A single pole response at the
crossover frequency yields a very stable loop with 90 de-
grees of phase margin.
For the design example, a target loop bandwidth (crossover
frequency) of 20 kHz was selected. The compensation net-
work zero (f
nitude less than the target crossover frequency. This con-
strains the product of R4 and C5 for a desired compensation
network zero (1 / (2π 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 kΩ. These values configure the compensation net-
work zero at 320 Hz. The error amp gain at frequencies
greater than f
Z
= 1 / (2πR4C5). The error amplifier zero cancels the
FIGURE 8. Gain and Phase of Modulator
R
Z
LOAD
) should be selected at least an order of mag-
Z
is: R4 / R5, which is approximately 10 (20dB).
= 5 Ohms and C
OUT
= 177 µF
(Continued)
20161915
17
The overall loop can be predicted as the sum (in dB) of the
modulator gain and the error amp gain.
If a network analyzer is available, the modulator gain can be
measured and the error amplifier gain can be configured for
the desired loop transfer function. If a network analyzer is not
available, the error amplifier compensation components can
be designed with the guidelines given. Step load transient
tests can be performed to verify acceptable performance.
The step load goal is minimum overshoot with a damped
response. C6 can be added to the compensation network to
decrease noise susceptibility of the error amplifier. The value
of C6 must be sufficiently small since the addition of this
capacitor adds a pole in the error amplifier transfer function.
This pole must be well beyond the loop crossover frequency.
A good approximation of the location of the pole added by C6
is: f
error amplifer noise susceptibility is to connect a capacitor
p2
= fz x C5 / C6. An alternative method to decrease the
FIGURE 9. Error Amplifier Gain and Phase
FIGURE 10. Overall Loop Gain and Phase
20161916
20161917
www.national.com