Lo w -Vo lt a g e , P re c is io n S t e p -Do w n
Co n t ro lle r fo r P o rt a b le CP U P o w e r
MAX136
V
CC
R1
R2
TO PWM
LOGIC
UNCOMPENSATED
HIGH-SPEED
FB
CC
LEVEL TRANSLATOR
AND BUFFER
OUTPUT DRIVER
I1
I2
I3
I4
V
BIAS
REF
CSH
CSL
SLOPE COMPENSATION
Figure 4. Main PWM Comparator Functional Diagram
b y re d uc ing the e ffe c tive fre que nc y, subse que ntly
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 fixe d -fre q ue nc y PWM op e ra tion a s loa d c urre nt
increases.
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.
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
/ V ). As the
OUT
IN
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.
S yn c h ro n o u s Re c t ifie r Drive r (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.
In PWM mode, the controller operates as a fixed-fre-
quency, current-mode controller in which the duty fac-
tor is s e t b y the inp ut/outp ut volta g e ra tio. 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.
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-
trolle d d e a d time to p re ve nt c ros s -c ond uc tion or
“shoot-through”). In discontinuous (light-load) mode,
the synchronous switch is turned off as the inductor
current falls through zero.
______________________________________________________________________________________ 13
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
