1.5A, 280kHz, Boost Regulator
APPLICATION INFORMATION
Current Mode Control
V
IN
Oscillator
V
C
-
R
+
PWM
Comparator
X5
63mΩ
S
Q
Power Switch
V
SW
L
D1
MBRS120T3
C
0
LM5171
R
Load
Figure 19. Current Mode Controle Scheme
The LM5171 is a current mode control scheme, in which the PWM ramp signal is derived from the power
switch current. This ramp signal is compared to the output of the error amplifier to control the on-time of the
power switch. The oscillator is used as a fixed-frequency clock to ensure a constant operational frequency.
The resulting control scheme features several advantages over conventional voltage mode control. First,
derived directly from the inductor, the ramp signal responds immediately to line voltage changes. This
eliminates the delay caused by the output filter and error amplifier, which is commonly found in voltage mode
controllers. The second benefit comes from inherent pulse−by−pulse current limiting by merely clamping the
peak switching current. Finally, since current mode commands an output current rather than voltage, the filter
offers only a single pole to the feedback loop. This allows both a simpler compensation and a higher
gain−bandwidth over a comparable voltage mode circuit.
Without discrediting its apparent merits, current mode control comes with its own peculiar problems, mainly,
sub harmonic oscillation at duty cycles over 50%. The LM5171 solves this problem by adopting a slope
compensation scheme in which a fixed ramp generated by the oscillator is added to the current ramp. A proper
slope rate is provided to improve circuit stability without sacrificing the advantages of current mode control.
Oscillator and Shutdown
Figure 20. Timing Diagram of Sync and Shutdown
Dec. 2010 - Rev. 1.2.1
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8
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HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
