MIC3838 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – Flexible Push-Pull PWM Controller

MIC3838/3839
Soft Start
The soft start feature helps reduce surge currents at the
power supply input source. An internal current source and
capacitor ramp up from 0V to near V_DD at a typical rate of
1V/ms. The soft start feature limits the output voltage of the
error amplifier at the COMP pin. As the soft start voltage rises,
it allows the COMP pin voltage to rise, which in turn allows the
duty cycle of the output drivers to increase. The internal soft
start voltage is discharged and remains discharged during
the following conditions:
1. The V_DD voltage drops below the turn-off
threshold.
2. The voltage on the CS pin exceeds the overcur-
rent comparator threshold.
Once the internal soft start discharge FET is turned on, it
cannot be turned off until the internal soft start voltage drops
down below 0.5V. This insures a clean restart.
Oscillator
The oscillator operates at twice the switching frequency of
either OUTA or OUTB. The oscillator generates a sawtooth
waveform on the RC pin. The rising edge of the waveform is
controlled by the external resistor/capacitor combination.
The fall time is set by the on-resistance of the discharge FET
(see Figure 3). The fall time sets the delay (dead time)
between the turn-off of one output driver and the turn-on of the
other driver. A toggle flip-flop insures that drive signals to
OUTA and OUTB are alternated and therefore insures a
maximum duty cycle of less than 50% for each output driver.
Graphs of component values vs. oscillator frequency and
dead time are shown in the typical characteristic section of
this specification.
V_DD
4
RC
VDD
2
0.2V
S
Q
R
OSCILLATOR
OUTPUT
Figure 3. Oscillator
The voltage source to the resistor/capacitor timing compo-
nents is V_DD. The internal turn-off comparator threshold in the
oscillator circuit is V_DD/2. This allows the oscillator to track
changes in V_DD and minimize frequency variations in the
oscillator. The oscillator frequency can be roughly approxi-
mated using the following formula:
F_OSCILLATOR
=
1.41
R×C
Where: frequency is in Hz
Resistance is in ohms
Capacitance is in Farads.
Micrel
Graphs of oscillator frequency and dead time vs component
values are shown in the Typical Characteristic section of this
specification. The recommended range of timing resistors
and capacitors is 7kΩ to 200kΩ and 100pF to 1000pF. To
minimize oscillator noise and insure a stable waveform the
following layout rules should be followed:
1. The higher impedance of capacitor values less
than 100pF may causes the oscillator circuit to
become more susceptible to noise. Parasitic pin
and etch trace capacitances become a larger
part of the total RC capacitance and may
influence the desired switching frequency.
2. The circuit board etch between the timing
resistor, capacitor, RC pin and ground must be
kept as short as possible to minimize noise
pickup and insure a stable oscillator waveform.
3. The ground lead of the capacitor must be routed
close to the ground lead of the MIC3838/9.
Current Sensing and Overcurrent Protection
The features are:
• Peak current limit
• Overcurrent limit
• Internal current sense discharge
• Front edge blanking
In current mode control, a PWM comparator uses the inductor
current signal and the error amplifier signal to determine the
operating duty cycle. In the MIC3838/9 the signal at the CS
pin is level shifted up before it reaches the PWM comparator
as shown in Figure 1. This allows operation of the error
amplifier and PWM comparator in a linear region.
There are two current limit thresholds in the MIC3838/9; peak
current limit and overcurrent limit. The normal operating
voltage at the I_LIM pin is designed less than these thresholds.
A pulse-by-pulse current limit occurs when the inductor
current signal at the I_LIM pin exceeds the peak current limit
threshold. The on-time is terminated for the remainder of the
switching cycle, regardless of whether OUTA or OUTB is
active.
If the signal at the I_LIM pin goes past the peak threshold and
exceeds the overcurrent limit threshold, the overcurrent limit
comparator forces the soft start node to discharge and
initiates a soft start reset.
An internal FET discharges the RAMP and I_LIM pins at the end
of the oscillator charge time. The FET turns on when the
voltage on the RC pin reaches the upper threshold (V_DD/2)
and remains on for the duration of the RC pin discharge time
and for typically 100ns after the start of the next on-time
period. The 100ns period at the beginning of the on-time
implements a front edge blanking feature that prevents false
triggering of the PWM comparator due to noise spikes on the
leading edge of the current turn-on signal. The front edge
blanking also sets the minimum on-time for OUTA and OUTB.
The timing diagram for the RAMP pin is shown in Figure 4.
August 2003
9
MIC3838/3839