MIC2182 Datasheet, PDF (10/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller Final Information

MIC2182
Functional Description
See “Applications Information” following this section for com-
ponent selection information and Figure 14 and Tables 1
through 5 for predesigned circuits.
The MIC2182 is a BiCMOS, switched-mode, synchronous
step-down (buck) converter controller. Current-mode control
is used to achieve superior transient line and load regulation.
An internal corrective ramp provides slope compensation for
stable operation above a 50% duty cycle. The controller is
optimized for high-efficiency, high-performance dc-dc con-
verter applications.
The MIC2182 block diagrams are shown in Figure 2a and
Figure 2b.
The MIC2182 controller is divided into 6 functions.
• Control loop
- PWM operation
- Skip-mode operation
• Current limit
• Reference, enable and UVLO
• MOSFET gate drive
• Oscillator and sync
• Soft start
Micrel
Control Loop
PWM and Skip Modes of Operation
The MIC2182 operates in PWM (pulse-width-modulation)
mode at heavier output load conditions. At lighter load condi-
tions, the controller can be configured to automatically switch
to a pulse-skipping mode to improve efficiency. The potential
disadvantage of skip mode is the variable switching fre-
quency that accompanies this mode of operation. The occur-
rence of switching pulses depends on component values as
well as line and load conditions. There is an external sync
function that is disabled in skip mode. In PWM mode, the
synchronous buck converter forces continuous current to
flow in the inductor. In skip mode, current through the inductor
can settle to zero, causing voltage ringing across the induc-
tor. Pulling the PWM pin (pin 2) low will force the controller to
operate in PWM mode for all load conditions, which will
improve cross regulation of transformer coupled, multiple
output configurations.
PWM Control Loop
The MIC2182 uses current-mode control to regulate the
output voltage. This method senses the output voltage (outer
loop) and the inductor current (inner loop). It uses inductor
current and output voltage to determine the duty cycle of the
buck converter. Sampling the inductor current removes the
inductor from the control loop, which simplifies compensa-
tion.
V_IN
CONTROL LOGIC AND
PULSE-WIDTH MODULATOR
V_DD
Reference _VIN
Q
RS
PWM
COMPARATOR
PWM Mode
to Skip
Mode
LOW
FORCES
SKIP MODE
0.024V
Current
Sense
Amp
RESET
CORRECTIVE
RAMP
COMP
3
C_COMP
R_COMP
Oscillator
MIC2182 [adj.] PWM Mode
Error
Amp
100k
A_V = 2
G_m = 0.2×10^-3
VDD
11
VIN
10
VBST
14
HSD
16
VSW
15
LSD
13
C_IN
4.7µF
D2
C_BST
Q2
L1
R_CS
D1
Q1
PGND
12
V_OUT
C_OUT
CSH
8
VOUT
9
R1
FB
7
R2
V_OUT = 1.245V ^{}1+ _RR21^{}
Figure 3. PWM Operation
MIC2182
10
June 2000