Micrel, Inc.
MIC5159
June 2006
8
M9999-062706
JA
= 25.71°C/W
The heatsink and MOSFET must have a combined
thermal resistance to meet the above criteria.
The typical thermal resistance from the junction to the
case (
JC
) of a TO-263 (D
2
pack) is 6°C/W. Adding
0.2°C/W for case to sink thermal resistance (
CS
), the
heatsink must have a sink to ambient thermal resistance
(
SA
) of:
SA
=
JA
– (
JC
+
CS
)
SA
= 25.71°C/W – (6°C/W + 0.2°C/W)
SA
= 19.51°C/W
According to the calculations, the heatsink must have a
SA
of 19.51°C/W or better.
For a full discussion of heat sinking and thermal effects
on voltage regulators, refer to the “
Regulator Thermals
”
section of Micrel’s
Designing with Low-Dropout Voltage
Regulators
handbook.
Short-Circuit Current Limit
The above thermal design calculations apply to normal
operation. In the case where the P-Channel MOSFET
must survive extended periods of short-circuit current,
another approach for thermal design must be
considered. Due to the fact that the MIC5159 delivers
constant current limiting, power dissipated by the
MOSFET is equal to the input voltage multiplied by the
maximum output current.
Figure 1 shows a simple, inexpensive circuit that allows
the current limiting to be re-entrant. This reduces power
dissipation in current limited conditions. As the output
voltage begins to drop, the differential voltage across the
input and output increases. This pulls the current sense
voltage lower, reducing the amount of output current to
maintain 50mV across the sense resistor. This reduction
in output current equates to a reduction in power
dissipation in the MOSFET. Figures 2 and 3 show a
comparison of linear current limiting versus the re-
entrant current limiting scheme implemented in Figure 1.
MIC5159-1.8BM6/YM6
R
VOUT
R
SENSE
C1
10µF
3.3 V
IN
R
VIN
Q1,2,3
Si4433DYx3
ISENSE
VIN
GATE
ADJ
C2
47µF
1.8 V
OUT
1.5
Figure 1. Re-Entrant Current Limit
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
OUTPUT VOLTAGE (V)
OUTPUT CURRENT (A)
Constant Current Limit
Re-Entrant
Current Limit
Figure 2. Output Voltage Characteristics
Re-Entrant Current Limit
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1 1.5
POWER DISSIPATION (W)
OUTPUT VOLTAGE (V)
Re-Entrant
Current Limiting
Constant Current
Limiting
Figure 3. Power Dissipation
vs. Output Voltage
Enable/Shutdown
The MIC5159 comes with an active-high enable pin that
allows the regulator to be disabled. Forcing the enable
pin low disables the regulator and sends it into a low off-
mode-current state. Forcing the enable pin high enables
the output voltage. This part is CMOS and the enable pin
cannot be left floating; a floating enable pin may cause
an indeterminate state on the output.
Output Capacitor
The MIC5159 requires an output capacitor to maintain
stability and improve transient response. Proper
selection is important to ensure proper operation. The
MIC5159 output capacitor selection is highly dependent
upon the components and the application. With a very
high gate charge (gate capacitance) MOSFET, the
output requires a much larger valued ceramic capacitor
for stability. As an alternative to a large valued ceramic
capacitor, a smaller-valued tantalum capacitor can be
used to provide stability. At higher load currents, lower
R
DS(ON)
MOSFETs are used; these MOSFETs typically
having much larger gate charge. If the application does
not require ultra-low-dropout voltage, smaller values of
ceramic capacitance may be used.
