Rainbow Electronics MAX8621Z User Manual
Page 14
MAX8621Y/MAX8621Z
Output Capacitor
The output capacitors, C7 and C9 in
Figure 3, are
required to keep the output voltage ripple small and to
ensure regulation loop stability. C7 and C9 must have
low impedance at the switching frequency. Ceramic
capacitors with X5R or X7R dielectric are highly recom-
mended due to their small size, low ESR, and small
temperature coefficients. Due to the unique feedback
network, the output capacitance can be very low. For
most applications, a 2.2µF capacitor is sufficient. For
optimum load-transient performance and very low out-
put ripple, the output capacitor value in µF should be
equal or larger than the inductor value in µH.
Feed-Forward Capacitor
The feed-forward capacitors, C
FF
(C6 and C8 in Figure
3), set the feedback loop response, control the switch-
ing frequency, and are critical in obtaining the best effi-
ciency possible. Choose a small ceramic X7R
capacitor with value given by:
Select the closest standard value to C
FF
as possible.
For BUCK2, C8, R3, and L1 are calculated using the
same methods.
LDO Output Capacitor and
Regulator Stability
Connect a 4.7µF ceramic capacitor between OUT1 and
ground, and a second 4.7µF ceramic capacitor
between OUT2 and ground for 300mA applications. For
150mA applications, 2.2µF ceramic capacitors can be
used for OUT1 and OUT2. Connect a 2.2µF ceramic
capacitor between OUT3 and ground, and a second
2.2µF ceramic capacitor between OUT4 and ground.
The LDO output capacitor’s (C
OUT
) equivalent series
resistance (ESR) affects stability and output noise. Use
output capacitors with an ESR of 0.1
Ω or less to ensure
stability and optimum transient response. Surface-
mount ceramic capacitors have very low ESR and are
commonly available in values up to 10µF. Connect
C
OUT_
as close to the IC as possible to minimize the
impact of PC board trace inductance.
Thermal Considerations
The MAX8621Y/MAX8621Z total power dissipation, P
D
,
is estimated using the following equations:
where P
IN(BUCK1)
is the input power for BUCK1,
η is the
step-down converter efficiency, and R
DC(INDUCTOR)
is
the inductor’s DC resistance.
For example, operating with V
IN
= 3.7V, V
BUCK1
= 1.376V,
V
BUCK2
= 1.8V, V
OUT1
= V
OUT2
= 2.6V, V
OUT3
= 1.8V,
V
OUT4
= 3V, I
BUCK1
= I
BUCK2
= 300mA, I
OUT1
= I
OUT2
=
330mA, I
OUT3
= I
OUT4
= 100mA, P
IN(BUCK1)
= 516mW
and
η = 80%, P
IN(BUCK2)
= 651mW and
η = 83%:
C
L
R
Siemens
6
1
1
10
=
×
Dual Step-Down DC-DC Power-Management ICs
for Portable Devices
14
______________________________________________________________________________________
P
P
P
P
P
P
P
P
P
I
R
P
P
I
R
D
LOSS BUCK
LOSS BUCK
LOSS OUT
LOSS OUT
LOSS OUT
LOSS OUT
LOSS BUCK
IN BUCK
BUCK
DC INDUCTOR
LOSS BUCK
IN BUCK
BUCK
DC
=
+
+
+
+
+
=
Ч
(
)
Ч
=
Ч
(
)
Ч
−
−
−
−
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
/
/
1
2
1
2
3
4
1
1
1
2
2
2
2
2
1
100
1
100
η
η
INDUCTOR
INDUCTOR
LOSS OUT
OUT
IN
OUT
LOSS OUT
OUT
IN
OUT
LOSS OUT
OUT
IN
OUT
LOSS OUT
OUT
IN
OUT
P
I
V
V
P
I
V
V
P
I
V
V
P
I
V
V
)
(
)
(
)
(
)
(
)
1
1
1
2
2
2
3
3
3
4
4
4
=
Ч
(
)
=
Ч
(
)
=
Ч
(
)
=
Ч
(
)
−
−
−
−
P
P
mW
P
mW
P
mW
P
mW
P
mW
P
mW
mW
mW
mW
mW
mW
mW
LOSS OUT
LOSS OUT
LOSS OUT
LOSS OUT
LOSS BUCK
LOSS BUCK
D
(
)
(
)
(
)
(
)
(
)
(
)
1
2
3
4
1
2
363
190
70
94
102
363
363
190
70
94
102
1182
=
=
=
=
=
=
=
+
+
+
+
+
=