MIC2169 Micrel Semiconductor, MIC2169 Datasheet - Page 6

• Manufacturer Part Number: MIC2169
• Description: 500 KHZ PWM SYNCHRONOUS BUCK CONTROL IC
• Manufacturer: Micrel Semiconductor

MIC2169
Functional Diagram
Functional Description
The MIC2169 is a voltage mode, synchronous step-down
switching regulator controller designed for high power with-
out 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
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
transconductance error amplifier via the feedback resistors
R3, and R2 and compared to a reference voltage at the non-
inverting 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 com-
parator is a 0 to 1V triangular waveform. The comparator
generates a rectangular waveform whose width t
to the time from the start of the clock cycle t
the triangle crosses the output waveform of the error ampli-
fier. To illustrate the control loop, let us assume the output
voltage drops due to sudden load turn-on, this would cause
M9999-111803
The MIC2169 is a voltage mode step-down regulator. The
0.8V
5V
Reference
Bandgap
C
IN
Digital Delay
Soft-Start &
Error
Amp
5V LDO
Counter
Enable
Error
Loop
V
5V
IN
C2
0.8V
BG Valid
V
5V
DD
Clamp &
Startup
Current
COMP
R1
C1
Ramp
Clock
Current Limit
Reference
RCS
CS
V
V
0
REF
REF
until t
Comparator
PWM
+3%
3%
ON
MIC2169 Block Diagram
1
, the time
is equal
Comparator
Current Limit
Comparator
Hys
6
Driver
Logic
the inverting input of the error amplifier which is divided down
version of V
causing the output voltage of the error amplifier to go high.
This will cause the PWM comparator to increase t
the top side MOSFET, causing the output voltage to go up
and bringing V
Soft-Start
The COMP/EN pin on the MIC2169 is used for the following
three functions:
For better understanding of the soft-start feature, let’s as-
sume V
un-grounding the COMP/EN pin. The COMP pin has an
internal 6.5 A current source that charges the external com-
pensation capacitor. As soon as this voltage rises to 180mV
(t = Cap_COMP
internal V
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 40 A and an internal 11-bit
counter starts counting which takes approximately 2ms to
complete. During counting, the COMP voltage is clamped at
GND
1. Disables the part by grounding this pin
2. External compensation to stabilize the voltage
3. Soft-start
High-Side
Low-Side
control loop
Driver
Driver
IN
5V
DD
= 12V, and the MIC2169 is allowed to power-up by
V
MIC2169
DD
OUT
linear regulator to power up and as soon as it
OUT
to be slightly less than the reference voltage
back in regulation.
SW
LSD
0.18V/8.5 A), the MIC2169 allows the
HSD
BOOST
FB
RSW
R2
4
R3
Q1
Q2
D1
C
BST
L1
November 2003
V
ON
C
OUT
OUT
time of
Micrel