MIC2186_05 Datasheet, PDF (9/15 Pages) Micrel Semiconductor – Low Voltage PWM Control IC

MIC2186
SKIP Mode Operation
This control method is used to improve efficiency at low
output loads. A block diagram of the MIC2186 SKIP mode is
shown in Figure 2. The power drawn by the MIC2186 control
IC is (I_VINA · V_INA )+ (I_VINP · V_INP). The power dissipated by
the IC can be a significant portion of the total output power
during periods of low output current, which lowers the effi-
ciency of the power supply. In SKIP mode the MIC2186
lowers the IC supply current by turning off portions of the
control and drive circuitry when the IC is not switching. The
disadvantage of this method is greater output ripple and
variable switching frequency. The soft start, HiDC and Sync
pins have no effect when operating in SKIP mode.
In SKIP mode, switching starts when the feedback voltage
drops below the lower threshold level of the hysteresis
comparator. The OutN pin goes high, turning on the N-
channel MOSFET, Q1. Current ramps up in the inductor until
either the current limit comparator or the hysteretic voltage
comparator turns off Q1’s gate drive. If the feedback voltage
exceeds the upper hysteretic threshold, Q1’s gate drive is
terminated. However, if the voltage at the CSH pin exceeds
the SKIP mode current limit threshold, it terminates the gate
drive for that switching cycle. The gate drive remains off for
a constant period at the end of each switching cycle. This off
time period is typically 1µs when the F/2 pin is low and 2µs
when the F/2 pin is high. Figure 3 shows some typical SKIP
mode switching waveforms.
V_IN
I_inductor
I_inductor
Gate Driver
I_inductor
I_inductor
V_COMP
Voltage
Divider
V_REF
Gate Drive at OutN
T_ON
T_PER
Figure 3. SKIP Mode Waveform
The SKIP mode current threshold limits the peak inductor
current per cycle. Depending on the input, output and circuit
parameters, many switching cycles can occur before the
feedback voltage exceeds the upper hysteretic threshold.
Once the voltage on the feedback pin exceeds the upper
hysteretic threshold the gate drive is disabled. The output
load discharges the output capacitance causing Vout to
decrease until the feedback voltage drops below the lower
threshold voltage limit. The switching converter then turns the
gate drive back on. While the gate drive is disabled, the
MIC2186 draws less IC supply current then while it is switch-
ing, thereby improving efficiency at low output loads. Figure
4 shows the efficiency improvement at low output loads when
SKIP mode is selected.
Micrel, Inc.
Low Current
Efficiency
80
PWM Mode
70
60
SKIP Mode
50
40
30
20
10
00 _{0.05 0.1 0.15 0.2 0.25}
INPUT CURRENT (A)
Figure 4.
The maximum peak inductor current depends on the skip
current limit threshold and the value of the current sense
resistor, Rsense. For a typical 50mV current limit threshold in
SKIP Mode, the peak inductor current is:
I_{INDUCTOR_pk} =
50mV
R_SENSE
The maximum output current is SKIP mode depends on the
input conditions, output conditions and circuit component
values. Assuming a discontinuous mode where the inductor
current starts from zero at each cycle, the maximum output
current is calculated below:
( ) ^IO(max)=
2.5 × 10^−3 × L × fs
2 × R_SENSE2 × V_O − η ×
V_IN
where:
Iomax is the maximum output current
Vo is the output voltage
Vin is the input voltage
L is the value of the boost inductor
fs is the switching frequency
η is the efficiency of the boost converter
Rsense it the value of the current sense resistor
2.5·10^-3 is a constant based on the SKIP mode
current threshold (50mV)²
PWM Operation
Figure 5 shows typical waveforms for PWM mode of opera-
tion. The gate drive signal turns on the external MOSFET
which allows the inductor current to ramp up. When the
MOSFET turns off, the inductor forces the MOSFET drain
voltage to rise until the boost diode turns on and the voltage
is clamped at approximately the output voltage.
April 2005
9
M9999-042205