MIC2182_04 Datasheet, PDF (12/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller

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
M9999-042204
12
April 22, 2004