MIC5191_06 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – Ultra High-Speed, High-Current Active Filter / LDO Controller

Micrel, Inc.
Referring to Application Hint 17, Designing PCB Heat
Sinks, the minimum amount of copper area for a D²pack
at 28.8°C/W is 2750mm² (or 0.426in²). The solid line
denotes convection heating only (2 oz. copper) and the
dotted line shows thermal resistance with 250LFM air-
flow. The copper area can be significantly reduced by
increasing airflow or by adding external heat sinks.
Figure 8. PC Board Heat Sink
Another important characteristic is the amount of gate
capacitance. Large gate capacitance can reduce
transient performance by reducing the ability of the
MIC5190 to slew the gate. It is recommended that the
MOSFET used has an input capacitance <10nF (C_ISS).
Source threshold specified in most MOSFET data sheets
refers to the minimum voltage needed to fully enhance
the MOSFET. Although for the most part, the MOSFET
will be operating in the linear region and the V_GS (gate-
MIC5191
source voltage) will be less than the fully enhanced V_GS,
it is recommended the VCC voltage has 2V over the
minimum V_GS and output voltage. This is due to the
saturation voltage of the MIC5191 output driver.
V_{CC1, 2} ≥ 2V + V_GS + V_OUT
For our example, with a 1.5V output voltage, our
MOSFET is fully enhanced at 4.5V_GS, our V_CC voltage
should be greater or equal to 8V.
Input Capacitor
Good input bypassing is important for improved perfor-
mance. Low ESR and low ESL input capacitors reduce
both the drain of the N-Channel MOSFET, as well as the
source impedance to the MIC5191. When a load
transient on the output occurs, the load step will also
appear on the input. Deviations on the input voltage will
be reduced by the MIC5191’s PSRR, but nonetheless
appear on the output. There is no minimum input
capacitance, but for optimal performance it is
recommended that the input capacitance be equal to or
greater than the output capacitance.
Output Capacitor
The MIC5191 is stable with any type or value of output
capacitor (even without any output capacitor!). This
allows the output capacitor to select which parameters of
the regulator are important. In cases where transient
response is the most important, low ESR and low ESL
ceramic capacitors are recommended. Also, the more
capacitance on the output, the better the transient
response.
V_IN
J1
+V_IN
12V
330µF 10µF 10µF 10µF
16V
100k
J2
EN
CSH
U1 MIC2198-BML
1µF
25V
6
VIN
12
HSD
IRF7821
2 _EN/UVLO
4
CSH
11
VSW
10
BST
0.1µF
V_OUT
V_OUT
100pF
10k
5 _VOUT
3
FB
8
LSD
11.5k
560pF
1 _COMP
VDD ⁷
GND
9
8.06k
IRF7821
D1
SD103BWS
2.2µF
10V
L1
1.8µH
CSH
V_OUT
CDEP134-1R8MC-H
10k
V_OUT
D2
1N5819HW
330µF
Tantalum
Figure 9. Post Regulator
22µF
10µF 10µF
MIC5191
OUT
VIN
ISENSE
GND
VCC1
VCC2
FB
COMP
10nF
12.4k
1V_OUT @10A
1µF
December 2006
11
M9999-122206