MIC2172/3172
Micrel
Block Diagram MIC3172
V
SW
Pin 7
V
I N
Pin 5
EN
Pin 4
2.3V
Anti-Sat.
100kHz
Osc.
Logic
Driver
Q1
D1
Reg.
Com-
parator
FB
Pin 3
1.24V
Ref.
Error
Amp.
Current
Amp.
S
GND
Pin 1
COMP
Pin 2
P
P
GND GND
1
2
Pin 6 Pin 8
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 limit-
ing greatly improves the power switch reliability and provides
automatic output current limiting. Finally, current-mode op-
eration provides automatic input voltage feed forward which
prevents instantaneous input voltage changes from disturb-
ing 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.
4-20
1997
MICREL [ MICREL SEMICONDUCTOR ]
