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| ISL8216M Datasheet, PDF (12/29 Pages) Intersil Corporation – Complete switch mode power supply in one package | |||
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ISL8216M
The selection of the resistor in series with the zener diode can be
calculated as:
RZ
=
V-----I--N-----â----V-----Z-
IZ
(EQ. 4)
Where:
⢠VZ is the zener diodeâs working voltage, nominal 5.1V.
⢠IZ is the zener diodeâs working reverse current, typically about
5mA.
Power dissipation rating should be taken into consideration when
selecting RZ.
Oscillator and Frequency Synchronization
The ISL8216M has an internally set fixed frequency of 300kHz.
By adding an external resistor (RT) between VIN and RTCT pin and
a capacitor (CT) between RTCT pin and SGND, the ISL8216M can
provide adjustable frequency from 200kHz to 600kHz. The time
constant of RT/CT determines the oscillator frequency. The
frequency setting curve is shown in Figure 20. Note that any
parasitic capacitance present on the RTCT pin adds to the
equivalent CT value and thus decreases the switching frequency.
500
450
CT OPEN
400
350
CT = 220pF
300
TABLE 2. SWITCHING FREQUENCY FOR OPTIMUM EFFICIENCY FOR
DIFFERENT INPUT AND OUTPUT VOLTAGES (Continued)
SWITCHING
VIN
VOUT FREQUENCY
(V)
(V)
(kHz)
RT
48
12
400
143kΩ
CT
open
54.9kΩ
220pF
64
12
400
143kΩ
open
54.9kΩ
220pF
80
12
350
267kΩ
open
73.2kΩ
220pF
48
24
450
95.3kΩ
open
43.2kΩ
220pF
64
24
400
143kΩ
open
54.9kΩ
220pF
80
24
350
267kΩ
open
73.2kΩ
220pF
Note that when the controller is disabled, the voltage at RTCT pin
rises up to the input voltage. Hence, the voltage rating of the CT
capacitor must be sufficient to support the maximum input
voltage.
The SYNC pin provides the function to synchronize the
ISL8216Mâs switching frequency to an external source. When
frequency synchronization is used, the time constant of RT/CT
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.
250
10
100
1k
R5 VALUE (kΩ)
FIGURE 19. RT AND CT vs SWITCHING FREQUENCY
Table 2 provides frequency selection for optimum efficiency at
typical VIN and VOUT conditions and corresponding RT and CT
values.
TABLE 2. SWITCHING FREQUENCY FOR OPTIMUM EFFICIENCY FOR
DIFFERENT INPUT AND OUTPUT VOLTAGES
SWITCHING
VIN
VOUT FREQUENCY
(V)
(V)
(kHz)
RT
24
5
300
open
CT
open
36
5
300
open
open
48
5
300
open
open
24
12
400
143kΩ
open
54.9kΩ
220pF
36
12
400
143kΩ
open
54.9kΩ
220pF
PHASE
SYNC
RT/CT
FIGURE 20. SYNCHRONIZATION OPERATION
Submit Document Feedback 12
FN8607.2
May 9, 2014
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