MIC2182 Datasheet, PDF (11/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller Final Information

MIC2182
A block diagram of the MIC2182 PWM current-mode control
loop is shown in Figure 3 and the PWM mode voltage and
current waveforms are shown in figure 5A. The inductor
current is sensed by measuring the voltage across the
resistor, R_CS. A ramp is added to the amplified current-sense
signal to provide slope compensation, which is required to
prevent unstable operation at duty cycles greater than 50%.
A transconductance amplifier is used for the error amplifier,
which compares an attenuated sample of the output voltage
with a reference voltage. The output of the error amplifier is
the COMP (compensation) pin, which is compared to the
current-sense waveform in the PWM block. When the current
signal becomes greater than the error signal, the comparator
turns off the high-side drive. The COMP pin (pin 3) provides
access to the output of the error amplifier and allows the use
of external components to stabilize the voltage loop.
Micrel
Skip-Mode Control Loop
This control method is used to improve efficiency at light
output loads. At light output currents, the power drawn by the
MIC2182 is equal to the input voltage times the IC supply
current (I_Q). At light output currents, the power dissipated by
the IC can be a significant portion of the total output power,
which lowers the efficiency of the power supply. The MIC2182
draws less supply current in skip mode by disabling portions
of the control and drive circuitry when the IC in not switching.
The disadvantage of this method is greater output voltage
ripple and variable switching frequency.
A block diagram of the MIC2182 skip mode is shown in Figure
4. Skip mode voltage and current waveforms are shown in
figure 5B.
CONTROL LOGIC AND
SKIP-MODE LOGIC
LOW-SIDE DRIVER
ONE SHOT
V_DD
Reference _VIN
Q
RS
Skip-Mode
Current
Limit
0.07V
V_IN
C_IN
VDD
11
VIN
10
4.7µF
D2
VBST
14
HSD
C_BST
16
Q2
VSW
L1
R_CS
15
LSD
13
Q1
PGND
12
V_OUT
C_OUT
ONE SHOT
Low
Comp
LOW
FORCES
PWM MODE
Hysteresis
Comp
±1%
–2%VBG
MIC2182 [adj.] Skip Mode
June 2000
Current
Sense
Amp
A_V = 2
CSH
8
VOUT
9
R1
FB
7
V_OUT = 1.245V ^{}1+ _RR21^{}
R2
Figure 4. Skip-Mode Operation
11
MIC2182