AIC3843 Datasheet, PDF (10/11 Pages) Analog Intergrations Corporation – Current-Mode PWM Controller

AIC3843
about 85mA.
Error Amplifier
A fully compensated error amplifier is provided
with inverting input and output externally
accessible. The non-inverting input is internally
biased at 2.5V. The converter output voltage is
usually divided down and connected to the
inverting input.
The output of the error amplifier is accessible for
external loop compensation, with an offset at two
diode drops (≅1.4V) and divided by three, before
connected to the inverting input of the current
sense comparator. This guarantees that no drive
pulse appears at the output (pin 6).
Oscillator
The oscillator frequency can be programmed
through the setting of timing components R_T and
C_T. Capacitor C_T is charged from the 5.0V
reference output through R_T to about 2.8V and
discharged to about 1.2V by the internal
discharge current. When C_T is discharged the
output (pin 6) must be in the low state, thus
producing a controlled amount of output
deadtime. Note that many values of R_T and C_T
can produce the same frequency but only one
combination will yield a specific output deadtime
at a given frequency.
Current Sense Comparator and PWM
Latch
The output switch of AIC3843 is initiated by the
oscillator and terminated when the peak inductor
current reaches the threshold level established by
the error amplifier output (pin 1). The AIC3843 is
operated at a current mode since the inductor
current is monitored cycle-by-cycle and decides
the duty cycle.
The inductor current is converted to a voltage by
inserting the ground referenced sense resistor R_S
in series with the source of output switch M1.
This voltage is monitored by the current sense
input (pin 3) and is compared to a level derived
from the error amplifier output. In the normal
operating conditions the peak inductor current is
controlled by the voltage at pin 1 where
IPK = ^{V pin 1 − 1 4V}
3RS
PWM Latch is used to ensure that only a single
pulse appears at the output during any given
oscillator cycle. However, a narrow spike on the
leading edge of the current waveform can usually
be observed and may cause the power supply to
exhibit an instability when the output is tightly
loaded.
Output Switch
The AIC3843 contains a single totem-pole output
stage that was specifically designed for direct
drive of power MOSFET. If any under voltage
lockout is detected, internal circuitry will keep the
output switch in a sinking current mode, no
external pull down resistor is needed.
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