lm5088mhx-2 National Semiconductor Corporation, lm5088mhx-2 Datasheet - Page 10

• Manufacturer Part Number: lm5088mhx-2
• Description: Wide Input Range Non-synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

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Error Amplifier and PWM
Comparator
The internal high gain error amplifier generates an error signal
proportional to the difference between the regulated output
voltage and an internal precision voltage reference (1.205V).
The output of the error amplifier is connected to the COMP
pin allowing the user to connect loop compensation compo-
nents. Generally a type II network, as illustrated in Figure 1,
is sufficient. This network creates a pole at DC, a mid-band
zero for phase boost and a high frequency pole for noise re-
duction. The PWM comparator compares the emulated cur-
rent signal from the RAMP generator to the error amplifier
output voltage at the COMP pin. A typical control loop gain/
phase plot is shown in performance curves section of this
document.
FIGURE 2. Basic Enable Configuration
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Ramp Generator
The ramp signal used for the pulse width modulator in current
mode control is typically derived directly from the buck switch
current. This signal corresponds to the positive slope portion
of the buck inductor current. Using this signal for the PWM
ramp simplifies the control loop transfer function to a single
pole response and provides inherent input voltage feed-for-
ward compensation. The disadvantage of using the buck
switch current signal for PWM control is the large leading
edge spike due to circuit parasitics which must be filtered or
blanked. Also, the current measurement may introduce sig-
nificant propagation delays. The filtering time, blanking time
and propagation delay limit the minimum achievable pulse
width. In applications where the input voltage may be rela-
tively large in comparison to the output voltage, controlling
small pulse widths and duty cycles is necessary for regulation.
The LM5088 utilizes a unique ramp generator which does not
actually measure the buck switch current but rather recon-
structs or emulates the signal. Emulating the inductor current
provides a ramp signal that is free of leading edge spikes and
measurement or filtering delays. The current reconstruction
is comprised of two elements; a sample & hold DC level and
an emulated current ramp.
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