MIC4690 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – 500kHz 1A SuperSwitcher Buck Regulator

MIC4690
Block Diagrams
SHDN
Internal
Regulator
500kHz
Oscillator
Thermal
Shutdown
Current
Limit
Com-
parator
Reset
Driver
Error
Amp
MIC4690 [adj.]
1.23V
Bandgap
Reference
V_IN
IN
V_OUT
=
V_REF

R1
R2
+ 1^{}
R1
=
R2


V_OUT
V_REF

− 1_
V_REF = 1.23V
SW
V_OUT
C_OUT
R1
FB
R2
Adjustable Regulator
Micrel
Functional Description
The MIC4690 is a variable duty cycle switch-mode regulator
with an internal power switch. Refer to the block diagrams.
Supply Voltage
The MIC4690 operates from a +4V to +30V (transients to
34V) unregulated input. Highest efficiency operation is from
a supply voltage around +12V. See the efficiency curve on
page 5.
Enable/Shutdown
The shutdown (SHDN) input is TTL compatible. A logic-low
enables the regulator. A logic-high shuts down the internal
regulator which reduces the current to typically 1.5µA when
V_SHDN = V_IN = 12V and 30µA when V_SHDN = 5V. See
“Shutdown Input Behavior: Shutdown Hysteresis.”
Feedback
Fixed-voltage versions of the regulator have an internal
resistive divider from the feedback (FB) pin. Connect FB
directly to the output voltage.
Adjustable versions require an external resistive voltage
divider from the output voltage to ground, center tapped to the
FB pin. See Figure 1b for recommended resistor values.
Duty Cycle Control
A fixed-gain error amplifier compares the feedback signal
with a 1.23V bandgap voltage reference. The resulting error
amplifier output voltage is compared to a 500kHz sawtooth
waveform to produce a voltage controlled variable duty cycle
output.
A higher feedback voltage increases the error amplifier
output voltage. A higher error amplifier voltage (comparator
inverting input) causes the comparator to detect only the
peaks of the sawtooth, reducing the duty cycle of the com-
parator output. A lower feedback voltage increases the duty
cycle. The MIC4690 uses a voltage-mode control architec-
ture.
Output Switching
When the internal switch is ON, an increasing current flows
from the supply V_IN, through external storage inductor L1, to
output capacitor C_OUT and the load. Energy is stored in the
inductor as the current increases with time.
When the internal switch is turned OFF, the collapse of the
magnetic field in L1 forces current to flow through fast
recovery diode D1, charging C_OUT.
Output Capacitor
External output capacitor C_OUT provides stabilization and
reduces ripple.
Return Paths
During the ON portion of the cycle, the output capacitor and
load currents return to the supply ground. During the OFF
portion of the cycle, current is being supplied to the output
capacitor and load by storage inductor L1, which means that
D1 is part of the high-current return path.
June 2001
7
MIC4690