LM25088MH-2EVAL National Semiconductor, LM25088MH-2EVAL Datasheet - Page 11

• Manufacturer Part Number: LM25088MH-2EVAL
• Description: BOARD EVAL FOR LM25088MH-2
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM25088MH-2EVAL

• Design Resources: LM(2)5088-1/2 QS Component Calculator
• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 7A
• Voltage - Input: 5.5 ~ 36V
• Regulator Topology: Buck
• Frequency - Switching: 250kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25088
• Silicon Manufacturer: National
• Silicon Core Number: LM25088
• Kit Application Type: Power Management - Voltage Regulator
• Application Sub Type: Buck Controller
• Kit Contents: Board
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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
11
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 LM25088 utilizes a unique ramp generator which does
not actually measure the buck switch current but rather re-
constructs 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 re-
construction is comprised of two elements; a sample & hold
DC level and an emulated current ramp.
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