RT8884B
44
DS8884B-01 September 2013
www.richtek.com
©
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
SET2 resistor network design : the QR mechanism
parameters need to be designed first. Initial QR_TH is
designed as 0.4 x LL x ICC-DY = 36mV and QR_Width
is designed as 1.11 x T
ON
. The ICCMAX is designed as
90A. By using the information, the two equations can
be listed by using multi-function pin setting mechanism
R1 = 75.8k
Ω
, R2 = 9.68k
Ω
.
No load offset function disabled. Just connect a 0
Ω
resistor from SET3 pin to ground.
(3) Protection Settings :
OVP/UVP protections : When VSEN pin voltage is
350mV more than VID, the OVP will be latched. When
V
SEN
pin voltage is 350mV less than VID, the UVP will
be latched.
TSEN and VR_HOT design : Using the following equation
to calculate related resistances for VR_HOT setting.
Choosing R1 = 100k
Ω
and an NTC thermistor R
NTC
(25
°
C)
= 100k
Ω
which its
β
=4485. When temperature is 100
°
C,
the R
NTC
(100
°
C) = 4.85k
Ω
.Then R
2
= 2.8k
Ω
can be
calculated.
R1 = 160k
Ω
, R2 = 9.77k
Ω
.
TSEN
CC
NTC(100 C)
R2
V
= V
= 1.887V
R2
R
// R1
°
×
⎡
⎤
+ ⎣
⎦
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P
D(MAX)
= (T
J(MAX)
−
T
A
) /
θ
JA
where T
J(MAX)
is the maximum junction temperature, T
A
is
the ambient temperature, and
θ
JA
is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125
°
C. The junction to
ambient thermal resistance,
θ
JA
, is layout dependent. For
WQFN-32L 4x4 package, the thermal resistance,
θ
JA
, is
27.8
°
C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at T
A
= 25
°
C
can be calculated by the following formula :
P
D(MAX)
= (125
°
C
−
25
°
C) / (27.8
°
C/W) = 3.6W for
WQFN-32L 4x4 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance,
θ
JA
. The derating curve in Figure 25 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Figure 25. Derating Curve of Maximum Power
Dissipation
(
)
R2
0.286 =
5
R1 R2
R1 R2
0.737 = 80
μ
A
R1 R2
×
+
×
×
+
R2
0.566 =
5
R1 R2
R1 R2
0.686 = 80 A
R1 R2
×
+
×
⎛
⎞
μ ×⎜
⎟
+
⎝
⎠
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0
25
50
75
100
125
Ambient Temperature (°C)
Ma
xi
mu
m P
o
w
e
r D
is
si
p
a
tio
n
(
W
)
Four-Layer PCB
set. By using above information, the two equations can
be listed by using multi-function pin setting mechanism
