MIC2169 Datasheet, PDF (6/14 Pages) Micrel Semiconductor – 500 KHZ PWM SYNCHRONOUS BUCK CONTROL IC

MIC2169
Functional Diagram
C_IN
V_IN
RCS
CS
5V LDO
5V
Bandgap
Reference
V_DD
5V
Current Limit
0.8V
Reference
BG Valid
5V
Soft-Start &
Digital Delay
Counter
0.8V
Enable
Error
Loop
Error
Amp
Clamp &
Startup
Current
Ramp
Clock
PWM
Comparator
V_REF +3%
V_REF 3%
COMP
Current Limit
Comparator
V_DD
High-Side
Driver
Driver
Logic
5V
Low-Side
Driver
HSD
BOOST
4Ω
SW RSW
LSD
Hys
Comparator
FB
R3
R2
MIC2169
GND
D1
Q1
C_BST
L1
Q2
C1
C2
R1
Micrel
V_OUT
C_OUT
MIC2169 Block 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
The MIC2169 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
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_ON 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 ampli-
fier. To illustrate the control loop, let us assume the output
voltage drops due to sudden load turn-on, this would cause
the inverting input of the error amplifier which is divided down
version of V_OUT 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 t_ON time of
the top side MOSFET, causing the output voltage to go up
and bringing V_OUT back in regulation.
Soft-Start
The COMP/EN pin on the MIC2169 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 V_IN = 12V, and the MIC2169 is allowed to power-up by
un-grounding the COMP/EN pin. The COMP pin has an
internal 6.5µA 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 MIC2169 allows the
internal V_DD 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
M9999-111803
6
November 2003