MIC2168_05 Datasheet, PDF (7/14 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

MIC2168
Micrel, Inc.
is reduced to 8.5µA 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 MIC2168 has
two hysteretic comparators that are enabled when V_OUT is
within ±3% of steady state. When the output voltage reaches
97% of programmed output voltage, then the g_m error amplifier
is enabled along with the hysteretic comparator. This point
onwards, the voltage control loop (g_m error amplifier) is fully
in control and will regulate the output voltage.
Soft-start time can be calculated approximately by adding
the following four time frames:
t1 = Cap_COMP × 0.18V/8.5µA
t2 = 12 bit counter, approx 2ms
t3 = Cap_COMP × 0.3V/8.5µA

t4 - _
V_OUT
V_IN


x
0.5
x
Cap_COMP
8.5µA
Soft-Start Time(Cap_COMP=100nF) = t1 + t2 + t3 +
t4 = 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms
Current Limit
The MIC2168 uses the R_DS(ON) of the top power MOSFET
to 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 MIC2168 discharges the COMP capacitor to
0.65V, resets the digital counter and automatically shuts off
the top gate drive, and the g_m error amplifier and the –3%
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 MIC2168 current limiting circuit.
V_IN
C2
C_IN
0
RCS
CS
HSD
LSD
Q1
MOSFET N
L1 Inductor
Q2
MOSFET N
V_OUT
C1
C_OUT
200µA
Figure 1. The MIC2168 Current Limiting Circuit
The current limiting resistor R_CS is calculated by the follow-
ing equation:
R_{CS =} ^{RDS(ON) Q1 × LL}
200µA
Equation (1)
I_{L =} I_{LOAD =}
1
2(Inductor Ripple Current)
where:
Inductor Ripple Current =
V_{OUT =}
(V_IN - V_OUT)
V_IN × F_SWITCHING × L
F_SWITCHING = 1MHz
200µA is the internal sink current to program the MIC2168
current limit.
The MOSFET R_DS(ON) varies 30% to 40% with temperature;
therefore, it is recommended to add a 50% margin to the load
current (I_LOAD) in the above equation to avoid false current
limiting due to increased MOSFET junction temperature rise.
It is also recommended to connect R_CS resistor directly to
the drain of the top MOSFET Q1, and the R_SW resistor to the
source of Q1 to accurately sense the MOSFETs R_DS(ON). A
0.1µF capacitor in parallel with R_CS should be connected to
filter some of the switching noise.
Internal V_DD Supply
The MIC2168 controller internally generates V_DD for self bias-
ing and to provide power to the gate drives. This V_DD supply
is generated through a low-dropout regulator and generates
5V from V_IN supply greater than 5V. For supply voltage less
than 5V, the V_DD linear regulator is approximately 200mV
in dropout. Therefore, it is recommended to short the V_DD
supply to the input supply through a 10Ω resistor for input
supplies between 2.9V to 5V.
MOSFET Gate Drive
The MIC2168 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_IN. Diode D2 is 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_DD bias supply. The 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 simultane-
ously flowing unimpeded through both MOSFETs.
MOSFET Selection
The MIC2168 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
low-side switches. For applications where V_IN < 5V, the internal
April 2005
7
M9999-040805