Rainbow Electronics MAX1625 - Manual (Page 14)

MAX1624/MAX1625
High-Speed Step-Down Controllers with
Synchronous Rectification for CPU Power
14 ______________________________________________________________________________________
voltage is the average of the AC voltage at the switching
node, which is adjusted and regulated by changing the
duty cycle of the MOSFET switches. Slope compensa-
tion is necessary to stabilize current-mode feedback
controllers with a duty cycle greater than 50%. Maximum
duty cycle is greater than 85% (see
Typical Operating
Characteristics ).
PWM Controller Block and Integrator
The heart of the current-mode PWM controller is a
multi-input open-loop comparator that sums three sig-
nals: the buffered feedback signal, the current-sense
signal, and the slope-compensation ramp. This direct-
summing configuration approaches ideal cycle-by-
cycle control over the output voltage. The output
voltage error signal is generated by an error amplifier
that compares the amplified feedback voltage to an
internal reference.
Each pulse from the oscillator sets the main PWM latch
that turns on the high-side switch for a period deter-
mined by the duty factor (approximately V
OUT / V IN ).
The current-mode feedback system regulates the peak
inductor current as a function of the output voltage
REF
REF2
N
FB
REF
CC2
CC1
PWROK
REF1
10k
40k
WINDOW
CONTROL AND
DRIVE LOGIC
OSCILLATOR
SLOPE
COMPENSATION
REF
AGND
V CC
FREQ
REF
REF1 REF2
CSL
CSH
BST
DH
LX
V DD
RESET
Q
Q
SET
DL
PGND
MAX1625
Figure 4. MAX1625 Simplified Block Diagram