MIC5159 Datasheet, PDF (8/23 Pages) Micrel Semiconductor – PROGRAMMABLE CURRENT LIMIT UCAP LDO REGULATOR CONTROLLER

MIC5159
The typical thermal resistance from the junction to the case
(θ_JC) of a TO-263 (D² pack) is 6°C/W. Adding 0.2°C/W for
case to sink thermal resistance (θ_CS), the heatsink must have
a sink to ambient thermal resistance (θ_SA) of:
θ_SA = θ_JA– (θ_JC + θ_CS)
θ_SA = 25.71°C/W – (6°C/W + 0.2°C/W)
θ_SA = 19.51°C/W
According to the calculations, the heatsink must have a θ_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 dissipa-
tion 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, reduc-
ing 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.
3.3 V_IN
C1
10µF
9 ^RVIN
10
9 ^RSENSE
25m
9 RVOUT
1k
Q1,2,3
Si4433DYx3
1.8 V_OUT 1.5A
MIC5159-1.8BM6
ISENSE GATE
VIN
VOUT
C2
47µF
Figure 1. Re-Entrant Current Limit
2.0
1.8
Constant Current Limit
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Re-Entrant
Current Limit
0
Micrel
OUTPUT CURRENT (A)
Figure 2. Output Voltage Characteristics
Re-Entrant Current Limit
5
4.5
Constant Current
Limiting
4
3.5
3
2.5
Re-Entrant
2
Current Limiting
1.5
1
0.5
0
0
0.5
1
1.5
OUTPUT VOLTAGE (V)
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 stabil-
ity and improve transient response. Proper selection is impor-
tant to ensure proper operation. The MIC5159 output capaci-
tor selection is highly dependent upon the components and
the application. With a very high gate charge (gate capaci-
tance) 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 capaci-
tor can be used to provide stability. At higher load currents,
lower R_DS(ON) MOSFETs are used; these MOSFETs typi-
cally having much larger gate charge. If the application does
not require ultra-low-dropout voltage, smaller values of ce-
ramic capacitance may be used.
Input Capacitor
An input capacitor of 1.0µF or greater is recommended when
the device is more than 4 inches away from the bulk AC
supply capacitance or when the supply is a battery. Small,
surface mount, ceramic capacitors can be used for bypassing
the input to the regulator, further improving the integrity of the
output voltage. Larger input capacitors may be required
depending on the impedance of the source and the output
load requirements.
M9999-041204
8
April 2004