7FN6792.1
December 1, 2011
and the MOSFET gate charge, Qg, the average output
current can be calculated from Equation 5:
To optimize noise immunity, bypass VDD to GND with a
ceramic capacitor as close to the VDD and GND pins as
possible.
VREF - The 5.00V reference voltage output. +1.0/-1.5%
tolerance over line, load and operating temperature. Bypass
to GND with a 0.1µF to 3.3µF capacitor to filter this output as
needed.
Functional Description
Features
The ISL884xAMBEPZ current mode PWM makes an ideal
choice for low-cost flyback and forward topology
applications. With its greatly improved performance over
industry standard parts, it is the obvious choice for new
designs or existing designs which require up dating.
Oscillator
The ISL884xAMBEPZ has a sawtooth oscillator with a
programmable frequency range to 2MHz, wh ich can be
program med wi th a resistor from V REF and a capacitor to
GND on the RTCT pin. (Please refer to Figure 4 for the
resistor and capacitance required for a given frequency.)
Soft-Start Operation
Soft-start must be implemented externally. One method,
illustrated in Figure 5, clamps the voltage on COMP.
The COMP pin is clamped to the voltage on capacitor C1
plus a base-emitter junction by transistor Q1. C1 is charged
from VREF through resistor R1 and the base current of Q1.
At power-up C1 is fully discharged, COMP is at ~0.7V, and
the duty cycle is zero. As C1 charges, the voltage on COMP
increases, and the duty cycle increases in proportion to the
voltage on C1. When COMP reaches the steady state
operating point, the control loop takes over and soft-start is
complete. C1 continues to charge up to VREF and no longer
affects COMP. During power-down, diode D1 quickly
discharges C1 so that the soft-start circuit is properly
initialized prior to the next power-on sequ ence.
Gate Drive
The ISL884xAMBEPZ is capable of sourcing and sinking 1A
peak current. To limit the peak current through the IC, an
optional external resistor may be placed between the
totem-pole output of the IC (OUT pin) and the gate of the
MOSFET. This small series resistor also damps any
oscillations caused by the resonant tank of the parasitic
inductances in the traces of the board and the FET’s input
capacitance.
Slope Compensation
For applications where the maximum duty cycle is less than
50%, slope compensation may be used to improve noise
immunity, particularly at lighter loads. The amount of slope
compensation required for noise immunity is determined
empirically, but is generally about 10% of the full scale
current feedback signal. For applications where the duty
cycle is greater than 50%, slope compensation is required to
prevent instability.
Slope compensation may be accomplished by summing an
external ramp with the current feedback signal or by
subtracting the external ramp from the voltage feedback
error signal. Adding the external ramp to the current
feedback signal is the more popular method.
From the small signal current-mode model [1] it can be
shown that the naturally-sampled modula to r gain, Fm,
without slope compensation, is in Equation 6.
where Sn is the slope of the sawtooth signal and tsw is the
duration of the half-cycle. W hen an external ramp is added,
the modulator gain becomes Equation 7:
where Se is slope of the external ramp and
The criteria for determining the correct amount of external
ramp can be determined by appropria tely setting the
damping factor of the double-pole located at the switching
frequency. The double-pole will be critically damped if the
Q-factor is set to 1, over-damped for Q < 1, and
under-damped for Q > 1. An under-damped condition may
result in current loop instability.
IOUT Qg f×=(EQ. 5)
FIGURE 5. SOFT-START
VREF
COMP
GND
ISL884xAMBEPZ
C1
Q1
D1 R1
Fm 1
SntSW
------------------
=(EQ. 6)
Fm 1
SnSe
+()tSW
------------------------------------ 1
mcSntSW
---------------------------
== (EQ. 7)
mc1Se
Sn
-------
+= (EQ. 8)
Q1
πmc1D–()0.5–()
-------------------------------------------------
=(EQ. 9)
ISL8840AMZ, ISL8841AMZ, ISL8842AMZ, ISL8843AMZ, ISL8844AMZ, ISL8845 AMZ