Datasheet

Micrel, Inc.
MIC29150/29300/29500/29750
December 2012
20
M9999-122012-B
For example, given an expected maximum ambient
temperature (T
A
) of 75°C with V
IN
= 3.3V, V
OUT
= 2.5V,
and I
OUT
= 1.5A, first calculate the expected P
D
using
Equation 3:
P
D
=(3.3V–2.5V)1.5A–(3.3V)(0.016A)=1.1472W
Eq. 3
Next, calcualte the junction temperature for the expected
power dissipation.
T
J
=(θ
JA
×P
D
)+T
A
=(56°C/W×1.1472W)+75°C
=139.24°C
Eq. 4
Now determine the maximum power dissipation allowed
that would not exceed the IC’s maximum junction
temperature (125°C) without the useof a heat sink by:
P
D(MAX)
= (T
J(MAX)
–T
A
)/θ
JA
= (125°C–75°C)/(56°C/W) = 0.893W
Eq. 5
Capacitor Requirements
For stability and minimum output noise, a capacitor on
the regulator output is necessary. The value of this
capacitor is dependent upon the output current; lower
currents allow smaller capacitors. The
MIC29150/29300/29500/29750 regulators are stable
with the following minimum capacitor values at full load,
as noted in Table 1:
Device
Full Load Capacitor
MIC29150
10µF
MIC29300
10µF
MIC29500
10µF
MIC29750
22µF
Table 1. Minimum Capacitor Values at Full Load
This capacitor need not be an expensive low ESR type:
aluminum electrolytics are adequate. In fact, extremely
low ESR capacitors may contribute to instability.
Tantalum capacitors are recommended for systems
where fast load transient response is important.
Where the regulator is powered from a source with high
AC impedance, a 0.1µF capacitor connected between
Input and GND is recommended. This capacitor should
have good characteristics to above 250kHz.
Minimum Load Current
The MIC29150–29750 regulators are specified between
finite loads. If the output current is too small, leakage
currents dominate and the output voltage rises. The
following minimum load current swamps any expected
leakage current across the operating temperature range,
as shown in Table 2:
Device
Minimum Load
MIC29150
5mA
MIC29300
7mA
MIC29500
10mA
MIC29750
10mA
Table 2. Minimum Load Currents
Adjustable Regulator Design
The adjustable regulator versions, MIC29xx2 and
MIC29xx3, allow programming the output voltage
anywhere between 1.25V and the 25V. Two resistors are
used. The resistor values are calculated by Equation 6:








−×=
1
1.240
V
RR
OUT
2
1
Eq. 6
where V
OUT
is the desired output voltage. Figure 4 shows
component definition. Applications with widely varying
load currents may scale the resistors to draw the
minimum load current required for proper operation (see
“Minimum Load Current” sub-section).