MIC2199_10 Datasheet, PDF (5/14 Pages) Micrel Semiconductor – 300kHz 4mm × 4mm Synchronous Buck Converter

Micrel, Inc.
Block Diagram
EN/UVLO
2
Control
Logic
V_DD
Reference
V_IN
Current
Limit
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
MIC2199.
V_OUT
C_OUT
PWM
RESET
CORRECTIVE
RAMP
Oscillator
COMP
1
C_COMP
R_COMP
MIC2199
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 MIC2199 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 MIC2199 block diagram is shown above.
The MIC2199 controller is divided into 5 functions.
• Control loop
• Current limit
• Reference, enable and UVLO
• MOSFET gate drive
• Oscillator
Control Loop
The MIC2199 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 MIC2199 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 MIC2199 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.
January 2010
5
M9999-011310