Micrel
MIC2172/3172
Functional Description
Refer to “Block Diagram MIC2172” and “Block Diagram
MIC3172.”
Internal Power
The MIC2172/3172 operates when V
IN
is
≥
2.6V (and
V
EN
≥
2.0V for the MIC3172). An internal 2.3V regulator
supplies biasing to all internal circuitry including a
precision 1.24V band gap reference.
The enable control (MIC3172 only) enables or disables
the internal regulator which supplies power to all other
internal circuitry.
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 MIC2172/3172 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 MIC2172/3172 by eliminating the
base to collector stored charge which would delay Q1’s
turnoff.
Compensation
Loop stability compensation of the MIC2172/3172 can
be accomplished by connecting an appropriate network
from either COMP to circuit ground (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.
April 2006
10
M9999-041806
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
