MIC2169A(2005) 查看數據表（PDF） - Micrel
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MIC2169A Datasheet PDF : 17 Pages
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MIC2169A
Functional Diagram
CIN
VIN
RCS
CS
5V LDO
5V
Bandgap
Reference
VDD
5V
0.8V
BG Valid
Current Limit
Reference
5V
Soft-Start &
Digital Delay
Counter
0.8V
Enable
Error
Loop
Error
Amp
Clamp &
Startup
Current
Ramp
Clock
PWM
Comparator
VREF +3%
VREF 3%
COMP/EN
Current Limit
Comparator
VDD
High-Side
Driver
Driver
Logic
5V
Low-Side
Driver
HSD
BOOST
2
SW RSW
LSD
Hys
Comparator
FB
R3
R2
MIC2169A
GND
D1
Q1
CBST
L1
1.4
1000pF
Q2
C1
C2
R1
Micrel
VOUT
COUT
MIC2169A Block Diagram
Functional Description
The MIC2169A is a voltage mode, synchronous step-down
switching regulator controller designed for high power without
the use of an external sense resistor. It includes an internal
soft-start function which reduces the power supply input surge
current at start-up by controlling the output voltage rise time,
a PWM generator, a reference voltage, two MOSFET drivers,
and short-circuit current limiting circuitry to form a complete
500kHz switching regulator.
Theory of Operation
The MIC2169A is a voltage mode step-down regulator. The
figure above illustrates the block diagram for the voltage
control loop. The output voltage variation due to load or line
changes will be sensed by the inverting input of the transcon-
ductance error amplifier via the feedback resistors R3, and
R2 and compared to a reference voltage at the non-invert-
ing input. This will cause a small change in the DC voltage
level at the output of the error amplifier which is the input to
the PWM comparator. The other input to the comparator is
a 0 to 1V triangular waveform. The comparator generates
a rectangular waveform whose width tON is equal to the
time from the start of the clock cycle t0 until t1, the time the
triangle crosses the output waveform of the error amplifier.
To illustrate the control loop, let us assume the output volt-
age drops due to sudden load turn-on, this would cause the
inverting input of the error amplifier which is divided down
version of VOUT to be slightly less than the reference voltage
causing the output voltage of the error amplifier to go high.
This will cause the PWM comparator to increase tON time of
the top side MOSFET, causing the output voltage to go up
and bringing VOUT back in regulation.
Soft-Start
The COMP/EN pin on the MIC2169A is used for the following
three functions:
1. Disables the part by grounding this pin
2. External compensation to stabilize the voltage
control loop
3. Soft-start
For better understanding of the soft-start feature, let’s as-
sume VIN = 12V, and the MIC2169A is allowed to power-up
by un-grounding the COMP/EN pin. The COMP pin has an
internal 8.5µA current source that charges the external com-
pensation capacitor. As soon as this voltage rises to 180mV
(t = Cap_COMP × 0.18V/8.5µA), the MIC2169A allows the
internal VDD linear regulator to power up and as soon as it
crosses the undervoltage lockout of 2.6V, the chip’s internal
oscillator starts switching. At this point in time, the COMP
pin current source increases to 40µA and an internal 11-bit
counter starts counting which takes approximately 2ms to
complete. During counting, the COMP voltage is clamped
at 0.65V. After this counting cycle the COMP current source
M9999-111803
6
June 2005
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