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参数资料
| 型号： | ISL8107EVAL2Z |
| 厂商： | Intersil |
| 文件页数： | 10/16页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD 2 FOR ISL8107 |
| 标准包装： | 1 |
| 系列： | * |
ISL8107
(EQ. 3)
F OSC = ------------------------------------------------------------------
0.1215R T C T + 140 × 10
R OCSET = --------------------------------------
The time constant of R T /C T determines the oscillator
frequency, which can be calculated through Equation 3:.
1
– 9
Note that when the controller is disabled, the voltage at
RT/CT pin rises up to the input voltage. Hence, the voltage
rating of the C T capacitor must be sufficient to support the
maximum input voltage.
The SYNC pin provides the ISL8107 to synchronize its
switching frequency to the fundamental frequency of the
input waveform. The rising edge of the input synchronization
signal is used to terminate the R T /C T ramp signal; therefore,
when frequency synchronization is used, the time constant
of R T /C T must be set longer than the period of the sync
signal. Tie the sync pin to ground when the sync feature is
not used.
PHASE
SYNC
RT/C T
FIGURE 11. SYNCHRONIZATION OPERATION
Overcurrent Protection
The overcurrent protection function protects the converter
from overcurrent conditions by monitoring the current flowing
through the MOSFET. OCP is implemented via a resistor
(R OCSET ) and a capacitor (C OCSET ) connecting the OCSET
pin and the drain of the MOSFET. An internal 104μA current
source develops a voltage across R OCSET which is then
compared with the voltage developed across the MOSFET
at turn on as measured at the PHASE pin. When the voltage
drop across the MOSFET exceeds the voltage drop across
the resistor, a OCP event occurs. C OCSET is placed in
parallel with R OCSET to smooth the voltage across R OCSET
in the presence of switching noise on the input bus.
A 200ns blanking period is used to reduce the current
sampling error due to leading-edge switching noise.
The OCP trip point varies mainly due to MOSFET r DS(ON)
variations and layout noise concerns. To avoid overcurrent
tripping in the normal operating load range, find the R OCSET
resistor from the following equations with:
1. The maximum r DS(ON) at the highest junction
temperature;
2. The minimum I OCSET from the specification table
Determine the overcurrent trip point greater than the
maximum output continuous current at maximum inductor
ripple current.
Simple OCP Equation
I OC ? r DS ( ON )
100 μ A
Detailed OCP Equation
? I
+ ----- ? ? r
---------------------------------------------------------------------- )
? OC_SOU 2 ?
MINIMUM ON-TIME
The ISL8107 requires the MOSFET to be turned on to a
minimum of 200ns (Typ). This minimum gate pulse width is
R OCSET =
Δ I
I OCSET
DS ( ON
(EQ. 6)
required to ensure proper samplings of the overcurrent
protection circuit.
V IN - V OUT V OUT
F SW ? L OUT
For low duty cycle applications, the switching frequency
must be selected to satisfy the condition shown in
Δ I = --------------------------------- ? ----------------
V IN
F OSC < ------------------ ? ----------------------
Equation 4:
V OUT 1
η ? V IN t on – min
(EQ. 4)
F SW = Regulator Switching Frequency
Where η is converter efficiency.
MINIMUM OFF-TIME
At the termination of the oscillator ’s ramp, there is a 190ns
time interval before the next ramp starts. This time interval
creates the minimum-off time of the PWM. This period
ensures that the boot capacitor is refreshed. Equation 5 can
be used to calculate the switching frequency to meet the
condition:
η ? V IN ? t off – max
F OSC < ? 1 – ------------------ ? ? ------------------------
? V OUT ? 1
?
10
(EQ. 5)
FN6605.0
October 29, 2008
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