MIC2169 Micrel Semiconductor, MIC2169 Datasheet - Page 7

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

0.65V. After this counting cycle the COMP current source is
reduced to 8.5 A and the COMP pin voltage rises from 0.65V
to 0.95V, the bottom edge of the saw-tooth oscillator. This is
the beginning of 0% duty cycle and it increases slowly
causing the output voltage to rise slowly. The MIC2169 has
two hysteretic comparators that are enabled when V
within 3% of steady state. When the output voltage reaches
97% of programmed output voltage then the g
fier is enabled along with the hysteretic comparator. This
point onwards, the voltage control loop (g
fully in control and will regulate the output voltage.
Soft-start time can be calculated approximately by adding the
following four time frames:
Current Limit
The MIC2169 uses the R
measure output current. Since it uses the drain to source
resistance of the power MOSFET, it is not very accurate. This
scheme is adequate to protect the power supply and external
components during a fault condition by cutting back the time
the top MOSFET is on if the feedback voltage is greater than
0.67V. In case of a hard short when feedback voltage is less
than 0.67V, the MIC2169 discharges the COMP capacitor to
0.65V, resets the digital counter and automatically shuts off
the top gate drive, and the g
hysteretic comparators are completely disabled and the soft-
start cycles restarts. This mode of operation is called the
“hiccup mode” and its purpose is to protect the down stream
load in case of a hard short. The circuit in Figure 1 illustrates
the MIC2169 current limiting circuit.
November 2003
MIC2169
Figure 1. The MIC2169 Current Limiting Circuit
t1 = Cap_COMP
t2 = 12 bit counter, approx 2ms
t3 = Cap_COMP
Soft-Start Time(Cap_COMP=100nF) = t1 + t2 + t3 +
t4 = 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms
t4
V
V
OUT
IN
0.5
DS(ON)
0.18V/8.5 A
0.3V/8.5 A
Cap_COMP
m
8.5 A
of the top power MOSFET to
error amplifier and the –3%
m
error amplifier) is
m
error ampli-
OUT
is
7
The current limiting resistor R
equation:
where:
200 A is the internal sink current to program the MIC2169
current limit.
The MOSFET R
therefore, it is recommended to add a 50% margin to the load
current (I
limiting due to increased MOSFET junction temperature rise.
It is also recommended to connect R
drain of the top MOSFET Q1, and the R
source of Q1 to accurately sense the MOSFETs R
0.1 F capacitor in parallel with R
filter some of the switching noise.
Internal V
The MIC2169 controller internally generates V
biasing and to provide power to the gate drives. This V
supply is generated through a low-dropout regulator and
generates 5V from V
voltage less than 5V, the V
mately 200mV in dropout. Therefore, it is recommended to
short the V
resistor for input supplies between 2.9V to 5V.
MOSFET Gate Drive
The MIC2169 high-side drive circuit is designed to switch an
N-Channel MOSFET. The block diagram in Figure 2 shows
a bootstrap circuit, consisting of D2 and CBST, supplies
energy to the high-side drive circuit. Capacitor CBST is
charged while the low-side MOSFET is on and the voltage on
the VSW pin is approximately 0V. When the high-side
MOSFET driver is turned on, energy from CBST is used to
turn the MOSFET on. As the MOSFET turns on, the voltage
on the VSW pin increases to approximately V
reversed biased and CBST floats high while continuing to
keep the high-side MOSFET on. When the low-side switch is
turned back on, CBST is recharged through D2. The drive
voltage is derived from the internal 5V V
nominal low-side gate drive voltage is 5V and the nominal
high-side gate drive voltage is approximately 4.5V due the
voltage drop across D2. An approximate 20ns delay between
the high- and low-side driver transitions is used to prevent
current from simultaneously flowing unimpeded through both
MOSFETs.
MOSFET Selection
The MIC2169 controller works from input voltages of 3V to
13.2V and has an internal 5V regulator to provide power to
turn the external N-Channel power MOSFETs for high- and
Inductor Ripple Current = V
F
R
I
L
SWITCHING
CS
LOAD
I
DD
LOAD
DD
R
Supply
) in the above equation to avoid false current
DS(ON) Q1
200 A
supply to the input supply through a 10
DS(ON)
2 Inductor Ripple Current
= 500kHz
IN
varies 30% to 40% with temperature;
I
supply greater than 5V. For supply
L
DD
CS
1
OUT
is calculated by the following
linear regulator is approxi-
Equation (1)
CS
CS
should be connected to
V
resistor directly to the
IN
DD
SW
V – V
F
bias supply. The
IN
SWITCHING
IN
resistor to the
. Diode D2 is
M9999-111803
DD
OUT
DS(ON)
for self
Micrel
. A
L
DD