MIC2168A Datasheet, PDF (8/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

Micrel
MIC2168A
Functional Description
The MIC2168A is a voltage mode, synchronous step-
down switching regulator controller designed for high
output power without the use of an external sense
resistor. It includes an internal soft-start function which
reduces the power supply input surge current at start-up
by controlling the output voltage rise time, a PWM
generator, a reference voltage, two MOSFET drivers,
and short-circuit current limiting circuitry to form a
complete 1MHz switching regulator.
Theory of Operation
The MIC2168A is a voltage mode step-down regulator.
The block diagram illustrates the voltage control loop.
The output voltage variation due to load or line changes
will be sensed by the inverting input of the
transconductance error amplifier via the feedback
resistors R3, and R2 and compared to a reference
voltage at the non-inverting input. This will cause a small
change in the DC voltage level at the output of the error
amplifier which is the input to the PWM comparator. The
other input to the comparator is a 0 to 1V triangular
waveform. The comparator generates a rectangular
waveform whose width t_ON is equal to the time from the
start of the clock cycle t₀ until t₁, the time the triangle
crosses the output waveform of the error amplifier. To
illustrate the control loop, let us assume the output
voltage drops due to sudden load turn-on, this would
cause the inverting input of the error amplifier which is
divided down version of V_OUT to be slightly less than the
reference voltage causing the output voltage of the error
amplifier to go high. This will cause the PWM
comparator to increase t_ON time of the top side
MOSFET, causing the output voltage to go up and
bringing V_OUT back in regulation.
Soft-Start
The COMP pin on the MIC2168A is used for the
following three functions:
1. Disables the part by grounding this pin
2. External compensation to stabilize the voltage
control loop
3. Soft-start
For better understanding of the soft-start feature, let’s
assume V_IN = 12V, and the MIC2168A is allowed to
power-up. The COMP pin has an internal 8.5μA current
source that charges the external compensation
capacitor. As soon as this voltage rises to 180mV (t =
Cap_COMP × 0.18V/8.5μA), the MIC2168A allows the
internal V_DD linear regulator to power up and as soon as
it crosses the undervoltage lockout of 2.6V, the chip’s
internal oscillator starts switching. At this point in time,
the COMP pin current source increases to 40μA and an
internal 11-bit counter starts counting which takes
approximately 2ms to complete. During counting, the
COMP voltage is clamped at 0.65V. After this counting
cycle the COMP current source is reduced to 8.5 A and
the COMP pin voltage rises from 0.65V to 0.95V, the
bottom edge of the saw-tooth oscillator. This is the
beginning of 0% duty cycle and it increases slowly
causing the output voltage to rise slowly. The MIC2168A
has two hysteretic comparators that are enabled when
V_OUT is within ±3% of steady state. When the output
voltage reaches 97% of programmed output voltage,
then the g_m error amplifier is enabled along with the
hysteretic comparator. This point onwards, the voltage
control loop (g_m error amplifier) is fully in control and will
regulate the output voltage.
Soft-start time can be calculated approximately by
adding the following four time frames:
t1 = Cap_COMP × 0.18V/8.5μA
t2 = 12 bit counter, approx 2ms
t3 = Cap_COMP × 0.3V/8.5μA
t4_{⎜⎜⎝⎛}
V_OUT
V_IN
⎟⎟⎠⎞
×
0.5
×
Cap_COMP
8.5μ.
Soft-Start Time(Cap_COMP = 100nF) = t1 + t2 +
t3 + t4 = 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms
Current Limit
The MIC2168A uses the R_DS(ON) of the top power
MOSFET to measure output current. Since it uses the
drain to source resistance of the power MOSFET, it is
not very accurate. This scheme is adequate to protect
the power supply and external components during a
fault condition by cutting back the time the top MOSFET
is on if the feedback voltage is greater than 0.67V. In
case of a hard short when feedback voltage is less than
0.67V, the MIC2168A discharges the COMP capacitor
to 0.65V, resets the digital counter and automatically
shuts off the top gate drive, and the g_m error amplifier
and the –3% hysteretic comparators are completely
disabled and the soft-start cycles restarts. This mode of
operation is called the “hiccup mode” and its purpose is
to protect the down stream load in case of a hard short.
The circuit in Figure 1 illustrates the MIC2168A current
limiting circuit.
January 2010
8
M9999-011510