MIC5159_06 Datasheet, PDF (8/23 Pages) Micrel Semiconductor – Programmable Current Limit μCap LDO Regulator Controller

Micrel, Inc.
θ_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 (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
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.
R_VOUT
3.3 V_IN
C1
10µF
R_VIN
R_SENSE
Q1,2,3
Si4433DYx3
1.8 V_OUT 1.5A
MIC5159-1.8BM6/YM6
ISENSE GATE
VIN
ADJ
C2
47µF
Figure 1. Re-Entrant Current Limit
MIC5159
2.0
Constant Current Limit
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Re-Entrant
Current Limit
0
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
00
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
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.
June 2006
8
M9999-062706