Datasheet

MAX1714
High-Speed Step-Down Controller
for Notebook Computers
______________________________________________________________________________________ 11
On-Time One-Shot (TON)
The heart of the PWM core is the one-shot that sets the
high-side switch on-time. This fast, low-jitter, adjustable
one-shot includes circuitry that varies the on-time in
response to battery and output voltage. The high-side
switch on-time is inversely proportional to the battery
voltage as measured by the V+ input, and proportional
to the output voltage. This algorithm results in a nearly
constant switching frequency despite the lack of a fixed-
frequency clock generator. The benefits of a constant
switching frequency are twofold: first, the frequency can
be selected to avoid noise-sensitive regions such as the
455kHz IF band; second, the inductor ripple-current
operating point remains relatively constant, resulting in
easy design methodology and predictable output volt-
age ripple.
On-Time = K (V
OUT
+ 0.075V) / V
IN
REF
-6%
FROM
OUT
REF
FB
ERROR
AMP
TOFF
TON
REF
+12%
REF
-30%
FEEDBACK
MUX
(SEE FIGURE 9)
CHIP
SUPPLY
x2
SHDN
PGOOD
MAX1714A ONLY
NOTE: IN THE MAX1714B, AGND AND PGND ARE INTERNALLY CONNECTED TO THE GND PIN.
( ) ARE FOR THE MAX1714B ONLY.
(MAX1714B ONLY)
OVP/UVLO
LATCH
ON-TIME
COMPUTE
TON
1-SHOT
1-SHOT
TRIG
IN
2V TO 28V
TRIG
Q
Q
S
R
2V
REF
AGND
REF
OUT
(GND)
(GND)
PGND
+5V
OUTPUT
DL
V
CC
V
DD
LX
ZERO CROSSING
CURRENT
LIMIT
DH
BST
9R
R
I
LIM
+5V
+5V
Q
S1
Q
S2 TIMER
SKIP
TON
V+
Σ
MAX1714
S
R
Q
Figure 2. MAX1714 Functional Diagram