Application Note 1907
2
AN1907.0
February 10, 2014
Evaluation Board Information
The evaluation board size is 76.2mmx76.2mm (3 inchx3 inch).
The FR4 board has four layers, with 2oz copper on the top and
the bottom layers, and 1oz copper on the two internal layers. The
board can be used as 4A reference design. Refer to the
“ISL8216MEVAL1Z Board Schematic” on page 4 and
“ISL8216MEVAL1Z Layouts” on page 5. All the components on
the board, including the solder attachment, are lead free.
Circuits Description
Selecting Output Voltage
The ISL8216MEVAL1Z evaluation board has several output voltages
preset for user’s convenience. The 5V, 12V, 20V, 24V, and 30V can
be easily selected by shorting their appropriate jumper. If any other
output voltage between 2.5V and 30V is desired, the user can use a
feedback resistor based on Equation 1.
NOTE: The output voltage accuracy is not only dependent on the module’s
internal reference voltage, but also on the feedback resistance accuracy.
For higher overall output accuracy, the customer should select a high
accuracy resistor (i.e; 0.5%).
Programming the Switching Frequency
The ISL8216M has an internally set fixed switching frequency of
300kHz. By adding a resistor between V
IN
and RTCT and a
capacitor between RTCT and SGND, the module’s switching
frequency can be adjusted between 200kHz to 600kHz. The
place holders for the resistor (R5) and capacitor (C5) are provided
on the ISL8216MEVAL1Z evaluation board for the user’s
convenience. Refer to the
ISL8216M
datasheet for optimum
switching frequency for different conditions.
The SYNC pin can provide the function to synchronize the
ISL8216M’s switching frequency to an external source. When
frequency synchronization is used, the time constant of R
T
/C
T
must be set longer than the period of the sync signal. When the
external sync feature is not used, the customer should tie the
SYNC pin to SGND.
Optional External Enable Control Circuit
The module can be enabled by an external signal by using an
open-drain device, or by adding an external circuit. This optional
external enable circuit is provided on the ISL8216MEVAL1Z
evaluation board, see Figure 2.
To utilize the circuit shown in Figure 2, the user can place
appropriate components on the place holders for R201, D201,
R202, Q201, and Q202.
A bias voltage of approximately 5.1V is generated from V
IN
with a
resistor (R201) in series and a zener diode (D201). R202 is a
pull-up resistor of typically 100k
Ω
, and Q201 and Q202 are
N-MOSFETs. The external control signal is applied to TP14
(EX_EN). When EX_EN is logic low, ENSS is pulled to ground to
disable the module. When EX_EN is logic high, ENSS is released
to allow the module’s soft-start function. The selection of R201 in
series with the zener diode D201 can be calculated as shown in
Equation 2:
Where:
• V
Z
is the zener diode D201’s working voltage, nominal 5.1V.
• I
Z
is the zener diode D201’s working reverse current, typically
about 5mA.
The power dissipation rating should be taken into consideration
when selecting R201.
Other External Circuits Requirements
Depending on the applications conditions, other external circuits
may be required. Table 1 shows these requirements.
V
OUT
1 11.3k
R
FB
----------------------------
+
1.192V
=
(EQ. 1)
FIGURE 2. OPTIONAL EXTERNAL ENABLE CIRCUIT
ON/OFF
BIAS VOLTAGE
ENSS
R20
2
V
IN
R2
0
1
D20
1
V
OUT
Q20
1
Q2
02
Q2
03
EX_EN
TP14
(EQ. 2)
R201
V
IN
V
Z
–
I
Z
----------------------------
=
TABLE 1. EXTERNAL CIRCUITS REQUIREMENT BASED ON APPLICATION CONDITIONS
CONDITIONS
EXTERNAL CIRCUITS REQUIREMENTS
V
OUT
Use PGOOD Signal
Enable Method
PGOOD Delay Circuit
1k
Ω
Dummy Load Resistor
V
IN
-BOOT Resistor
≤
12V
Yes/No
Self or External Enable Control
No
No
No
>12V
Yes
Self Enable
Yes
Yes
No
>12V
No
Self Enable
No
No
Yes
>12V
Yes
External Enable Control
No
No
Yes
>12V
No
External Enable Control
No
No
Yes
