AN45 SILABS [Silicon Laboratories], AN45 Datasheet - Page 6

• Manufacturer Part Number: AN45
• Description: DESIGN GUIDE FOR THE Si3210/15/16 DC-DC CONVERTER
• Manufacturer: SILABS [Silicon Laboratories]

AN45
on-time and the more efficient the converter will be
since the base current is not delivered to the load.
Also, as more base current is required, it becomes more
difficult for the DCDRV and DCFF pins to switch the
transistor off quickly, which further decreases efficiency.
Practically, Q7 gain should be around 100 at peak
inductor current.
Si321x Bipolar Switch Driver
In Figure 2, the Q8 collector current provides the base
current drive that turns the switch transistor Q7 on. The
base current drive should be sufficient to keep Q7 in
saturation at I
The Si321x sets the DCDRV pin high to create the base
current drive through Q8. The values of R17 and V
control the base drive current.
Transistor requirements for Q8:
V
V
V
f
Selecting Output Capacitor and Filter
The output capacitor, C9, is subject to large ac currents
from the inductor and should have low equivalent series
resistance (ESR) to minimize ripple voltage on V
A 10 µF, 100 V electrolytic capacitor provides adequate
filtering in most applications.
An RC filter between C9 and the load reduces ripple
voltage on the V
6
T
CEO
EBO
CBO
> 200 MHz
> V
> V
> V
CC
CC
CC
+ V
+ V
PK
I
I
BQ7
BQ7
V
. The overdrive factor of 1.3 is sufficient.
DC
DC
R
RIPPLE
I
--------------- -
BAT
CMAX
17
I
BQ7
=
=
=
Equation 17
Equation 18
Equation 19
output. This filter should have a
1.3
V
-------------------------------
--------------------------------------------- -
I
<
BQ7
CC
=
h
×
V
R
FEmin@ICmax
ERS
–
CC
------------------------------------- -
h
17
+
0.6 V
FEmin@ICmax
0.6 V R
–
I
MAX
0.7 V
CMAX
–
⁄
×
0.6 V
------------- -
R
I
16
PK
16
BAT
.
CC
Rev. 0.5
time constant of less than 100 µs to maintain loop
stability.
(Recommended values for these two components are
.1 µF and 15 Ω, respectively.)
Undervoltage and Overcurrent Protection
The Si321x dc-dc converter is designed to operate
under a specific input voltage and output loading
condition. When the input voltage goes too low, there is
not enough power from the input to deliver to the output;
so, the dc-dc converter may try to draw excessive
current in an attempt to deliver power to the output. A
similar condition exists when the output ramps up too
fast (during power up, transient loading), short loads, or
unintentionally overloads the output. To prevent
damage to the switching transistor during these
abnormal conditions, the Si321x implements an
undervoltage and overcurrent mechanism.
Undervoltage Lock-Out
The undervoltage lock-out is implemented via the
SDCH pin as shown in Figure 5. When the V
under a specified value, the current flow through R19
into the SDCH pin goes under 120 µA, and it triggers
the Si321x to shut off the dc-dc converter. The equation
for R19 with a specific V
Figure 5. Protection Sense Circuitry
0.8 V
0.8 V
See
4.5 k
4.5 k
DC
DC
R
19
CFILT
=
Equation 20
--------------------------- 4.5 kΩ
VDC
------------ - 0.8
Pin 8
Pin 9
1.5
SDCH
SDCL
120 µA
DC
and
–
is given by the following:
R19
R20
–
RFILT
Q7
VDC
in
R18
Figure 2.
DC
I
C
goes