MIC5159BM6 TR【PDF 8/23 页】IC REG CTRLR SGL POS ADJ SOT23-6

Micrel, Inc.

MIC5159

June 2006

8

M9999-062706

?/DIV>

JA

= 25.71癈/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 (?/DIV>

JC

) of a TO-263 (D

2

pack) is 6癈/W. Adding

0.2癈/W for case to sink thermal resistance (?/DIV>

CS

), the

heatsink must have a sink to ambient thermal resistance

(?/DIV>

SA

) of:

?/DIV>

SA

= ?/DIV>

JA

(?/DIV>

JC

+ ?/DIV>

CS

)

?/DIV>

SA

= 25.71癈/W (6癈/W + 0.2癈/W)

?/DIV>

SA

= 19.51癈/W

According to the calculations, the heatsink must have a

?/DIV>

SA

of 19.51癈/W or better.

For a full discussion of heat sinking and thermal effects

on voltage regulators, refer to the Regulator Thermals

section of Micrels 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

10礔

3.3 V

IN

R

VIN

Q1,2,3

Si4433DYx3

ISENSE

VIN

GATE

ADJ

C2

47礔

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

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

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.

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