
thermal resistance of the package chosen, and
θ
CA
is
the thermal resistance from the case through the PC
board, copper traces, and other materials to the sur-
rounding air. Figure 5 shows the allowable power dissi-
pation for typical PC boards at +25
°
C, +50
°
C, and
+70
°
C ambient temperatures.
The MAX8869 TSSOP-EP package features an
exposed thermal pad on its underside. This pad lowers
the package
’
s thermal resistance by providing a direct
thermal heat path from the die to the PC board.
Additionally, the ground pin (GND) also channels heat.
Connect the exposed thermal pad and GND to circuit
ground by using a large pad (1in
2
minimum recom-
mended) or multiple vias to the ground plane.
Applications Information
Capacitor Selection and
Regulator Stability
Capacitors are required at the MAX8869 input and out-
put. Connect 1μF or greater capacitors between IN and
GND (C
IN
) and OUT and GND (C
OUT
). Due to the
MAX8869
’
s relatively high bandwidth, use only surface-
mount ceramic capacitors that have low equivalent
series resistance (ESR) and high self-resonant frequen-
cy (SRF). Make the input and output traces at least
2.5mm wide (the width of the four parallel pins), and
connect C
IN
and C
OUT
within 6mm of the IC to mini-
mize the impact of PC board trace inductance. The
width of the ground trace should be maximized under-
neath the IC to ensure a good connection between pin
10 (GND) and the ground side of the capacitors.
The output capacitor
’
s ESR and SRF can affect stability
and output noise. Use capacitors with greater than 5MHz
SRF and ESR of 60m
or less to ensure stability and
optimum transient response. This is particularly true in
applications with very low output voltage (<2V) and high
output current (>0.5A).
Since some capacitor dielectrics may vary over bias
voltage and temperature, consult the capacitor manu-
facturer specifications to ensure that the capacitors
meet these requirements over all voltage and tempera-
ture conditions used.
Soft-Start Capacitor Selection
A capacitor (C
SS
) connected from SS to GND causes
the MAX8869 output current to slowly rise during start-
up, reducing stress on the input supply. The rise time to
full current limit (t
SS
) is determined by:
t
SS
= 2.08
10
-4
C
SS
where C
SS
is in nF. Typical capacitor values between
10nF to 100nF, with a 5V rating, are sufficient.
Because this ramp is applied to the current-limit com-
parator, the actual time for the output voltage to ramp
up depends on the load current and output capacitor.
Leave SS open to disable soft-start.
Input-Output (Dropout) Voltage
A regulator
’
s minimum input-to-output voltage differen-
tial (dropout voltage) determines the lowest usable sup-
ply voltage. In battery-powered systems, this
determines the useful end-of-life battery voltage. Since
a 0.2
P-channel MOSFET is used as the pass device,
dropout voltage is the product of R
DS(ON
) and load
current (see
Electrical Characteristics
and Dropout
Voltage vs. Output Current in
Typical Operating
Characteristics
). The MAX8869 operating current
remains low in dropout.
Noise, PSRR, and Transient Response
The MAX8869 is designed to achieve low dropout volt-
age and low quiescent current in battery-powered sys-
tems while still maintaining good noise, transient
response, and AC rejection (see PSRR vs. Frequency
in the
Typical Operating Characteristics)
. When operat-
ing from very noisy sources, supply noise rejection and
transient response can be improved by increasing the
values of the input and output capacitors and employ-
ing passive postfiltering. MAX8869 output noise is typi-
cally 150μV
RMS.
(see the Output Noise plot in the
Typical Operating Characteristics)
.
M
1A, MicroCap, Low-Dropout,
Linear Regulator
_______________________________________________________________________________________
9
Figure 5. Power Operating Region: Maximum Output Current
vs. Input-Output Differential Voltage
0
0.6
0.4
0.2
0.8
1.0
1.2
1
0
3
2
4
5
INPUT-OUTPUT DIFFERENTIAL VOLTAGE
M
MAXIMUM CONTINUOUS CURRENT
T
T
A
= +70
°
C
T
A
= +50
°
C
T
A
= +25
°
C
T
J
= +150
°
C
T
V
Chip Information
TRANSISTOR COUNT: 1088