MAX1636 Datasheet, PDF (13/24 Pages) Maxim Integrated Products – Low-Voltage, Precision Step-Down Controller for Portable CPU Power

Low-Voltage, Precision Step-Down
Controller for Portable CPU Power
V_CC
R1
R2
CC
FB
I1
I2
I3
I4
REF
CSH
CSL
SLOPE COMPENSATION
Figure 4. Main PWM Comparator Functional Diagram
TO PWM
LOGIC
UNCOMPENSATED
HIGH-SPEED
LEVEL TRANSLATOR
AND BUFFER
OUTPUT DRIVER
V_BIAS
by reducing the effective frequency, subsequently
reducing switching losses. It forces the peak inductor
current to ramp to 30% of the full current limit, deliver-
ing extra energy to the output and allowing subsequent
cycles to be skipped. Idle Mode transitions seamlessly
to fixed-frequency PWM operation as load current
increases.
With SKIP = high, the controller always operates in
fixed-frequency PWM mode for lowest noise. Each
pulse from the oscillator sets the main PWM latch that
turns on the high-side switch for a period determined
by the duty factor (approximately V_OUT / V_IN). As the
high-side switch turns off, the synchronous rectifier
latch sets; 60ns later, the low-side switch turns on. The
low-side switch stays on until the beginning of the next
clock cycle.
In PWM mode, the controller operates as a fixed-fre-
quency, current-mode controller in which the duty fac-
tor is set by the input/output voltage ratio. The
current-mode feedback system regulates the peak
inductor current value as a function of the output volt-
age error signal. In continuous-conduction mode, the
average inductor current is nearly the same as the
peak current, so the circuit acts as a switch-mode
transconductance amplifier. This pushes the second
output LC filter pole, normally found in a duty-factor-
controlled (voltage-mode) PWM, to a higher frequency.
To preserve inner-loop stability and eliminate regenera-
tive inductor current “staircasing,” a slope-compensa-
tion ramp is summed into the main PWM comparator to
make the apparent duty factor less than 50%.
The relative gains of the voltage-sense and current-
sense inputs are weighted by the values of current
sources that bias four differential input stages in the
main PWM comparator (Figure 4). The voltage sense
into the PWM has been conditioned by an integrated
component of the feedback voltage, yielding excellent
DC output voltage accuracy. See the Output Voltage
Accuracy section for more information.
Synchronous Rectifier Driver (DL)
Synchronous rectification reduces conduction losses in
the rectifier by shunting the normal Schottky catch
diode with a low-resistance MOSFET switch. Also, the
synchronous rectifier ensures proper start-up of the
boost gate-driver circuit. If the synchronous power
MOSFET is omitted for cost or other reasons, replace it
with a small-signal MOSFET, such as a 2N7002.
If the circuit is operating in continuous-conduction
mode, the DL drive waveform is simply the comple-
ment of the DH high-side-drive waveform (with con-
trolled dead time to prevent cross-conduction or
“shoot-through”). In discontinuous (light-load) mode,
the synchronous switch is turned off as the inductor
current falls through zero.
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