MIC2171_07 Datasheet, PDF (5/12 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator

Micrel, Inc.
Functional Diagram
MIC2171
IN
2.3V
Reg.
100kHz
Osc.
Logic
FB
Error
1.24V _Amp.
Ref.
Com-
parator
D1
SW
Anti-Sat.
Driver
Q1
Current
Amp.
COMP
GND
Functional Description
Refer to “Block Diagram MIC2171”.
Internal Power
The MIC2171 operates when V_IN is ≥ 2.6V. An internal
2.3V regulator supplies biasing to all internal circuitry
including a precision 1.24V band gap reference.
PWM Operation
The 100kHz oscillator generates a signal with a duty
cycle of approximately 90%. The current-mode
comparator output is used to reduce the duty cycle when
the current amplifier output voltage exceeds the error
amplifier output voltage. The resulting PWM signal
controls a driver which supplies base current to output
transistor Q1.
Current-Mode Advantages
The MIC2171 operates in current mode rather than
voltage mode. There are three distinct advantages to
this technique. Feedback loop compensation is greatly
simplified because inductor current sensing removes a
pole from the closed loop response. Inherent cycle-by-
cycle current limiting greatly improves the power switch
reliability and provides automatic output current limiting.
Finally, current-mode operation provides automatic input
voltage feed forward which prevents instantaneous input
voltage changes from disturbing the output voltage
setting.
Anti-Saturation
The anti-saturation diode (D1) increases the usable duty
cycle range of the MIC2171 by eliminating the base to
collector stored charge which would delay Q1’s turnoff.
Compensation
Loop stability compensation of the MIC2171 can be
accomplished by connecting an appropriate network
from either COMP to circuit ground (see “Typical
Applications”) or COMP to FB.
The error amplifier output (COMP) is also useful for soft
start and current limiting. Because the error amplifier
output is a transconductance type, the output impedance
is relatively high which means the output voltage can be
easily clamped or adjusted externally.
May 2007
5
M9999-051107