October 2005 5 MIC2198
MIC2198 Micrel, Inc.
Block Diagram
G
m
= 0.2×10
-3
Current
Limit
Error
V
IN
V
DD
CORRECTIVE
RAMP
RESET
Reference
Oscillator
EN/UVLO
2
7
10
6
12
11
8
9
4
5
3
1
COMP
100k
MIC2198
FB
R2
R1
VOUT
CSH
VDD
BST
C
IN
VIN
HSD
Q2
Q1
D1
D2
C
BST
C
OUT
V
IN
V
OUT
L1
VSW
LSD
PGND
C
COMP
R
COMP
4.7µF
A
V
= 2
Current
R
CS
Control
PWM
Logic
Amp
Sense
Amp
Figure 1. Internal Block Diagram
Functional Description
The MIC2198 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
converter applications.
The MIC2198 block diagram is shown above.
The MIC2198 controller is divided into 5 functions.
• Control loop
• Current limit
• Reference, enable and UVLO
• MOSFET gate drive
• Oscillator
Control Loop
The MIC2198 operates in PWM (pulse-width-modulation)
mode. In PWM mode, the synchronous buck converter
forces continuous current to flow in the inductor which also
improves cross regulation of transformer coupled, multiple
output configurations.
PWM Control Loop
The MIC2198 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 compensation.
A block diagram of the MIC2198 PWM current-mode control
loop is shown in Figure 2 and the PWM mode voltage and
current waveform is shown in Figure 3. The inductor current
is sensed by measuring the voltage across the resistor, R
CS
.
