MIC2198 Datasheet, PDF (6/15 Pages) Micrel Semiconductor – 500kHz 4mm × 4mm Synchronous Buck Controller

MIC2198
Block Diagram
EN/UVLO
2
Control
Logic
V_DD
Reference
V_IN
Current
Limit
Micrel
V_IN
VDD
7
VIN
6
BST
10
HSD
12
Q2
VSW
11
LSD
8
Q1
PGND
9
C_IN
D2
C_BST
L1
D1
4.7µF
R_CS
V_OUT
C_OUT
PWM
RESET
CORRECTIVE
RAMP
Oscillator
COMP
1
C_COMP
R_COMP
MIC2198
Error
Amp
100k
Current
Sense
Amp
A_V = 2
G_m = 0.2×10^-3
CSH
4
VOUT
5
R1
FB
3
V_OUT
=
0.8V
⎛⎝₁₊
R1 ⎞
R2 ⎠
R2
V_OUT(max) = 6.0V
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 con-
verter 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 con-
figurations.
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 compensa-
tion.
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.
MIC2198
6
November 2004