MIC2182 Datasheet, PDF (12/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller Final Information

MIC2182
V_IN
V_SW
0V
I_L1
Reset
Pulse
V_HSD
I_LOAD
0A
V_DD
0V
V_IN + V_DD
V_LSD
0V
V_DD
0V
Figure 5a. PWM-Mode Timing
Micrel
V_HSD
V_LSD
V_SW
I_L1
V_one-shot
V_OUT
V_DD
0V
V_DD
0V
V_IN
V_OUT
0V
I_LIM(skip)
0A
V_DD
0V
V_NOMI+N1A%L
–1%
0V
I_OUT
0A
Figure 5b. Skip-Mode Timing
A hysteretic comparator is used in place of the PWM error
amplifier and a current-limit comparator senses the inductor
current. A one-shot starts the switching cycle by momentarily
turning on the low side MOSFET to insure the high side drive
boost capacitor, Cbst, is fully charged. The high side MOS-
FET is turned on and current ramps up in the inductor, L1.
The high side drive is turned off when either the peak voltage
on the input of the current-sense comparator exceeds the
threshold, typically 35mV, or the output voltage rises above
the hysteretic threshold of the output voltage comparator.
Once the high side MOSFET is turned off, the load current
discharges the output capacitor, causing V_OUT to fall. The
cycle repeats when V_OUT falls below the lower threshold, –
1%.
The maximum peak inductor current depends on the skip-
mode current-limit threshold and the value of the current-
sense resistor, R_CS.
I_{inductor(peak)}
=
35mV
R_sense
Figure 6 shows the improvement in efficiency that skip mode
makes when at lower output currents.
100
80
Skip
60
PWM
40
20
00.01 0.1
1
10 100
OUTPUT CURRENT (A)
Figure 6. Efficiency
MIC2182
12
June 2000