Rainbow Electronics MAX1718 - Manual (Page 12)

MAX1718
Notebook CPU Step-Down Controller for Intel
Mobile Voltage Positioning (IMVP-II)
12 ______________________________________________________________________________________
Detailed Description
5V Bias Supply (V CC and V DD )
The MAX1718 requires an external 5V bias supply in
addition to the battery. Typically, this 5V bias supply is
the notebook’s 95% efficient 5V system supply.
Keeping the bias supply external to the IC improves
efficiency and eliminates the cost associated with the
5V linear regulator that would otherwise be needed to
supply the PWM circuit and gate drivers. If stand-alone
capability is needed, the 5V supply can be generated
with an external linear regulator.
The 5V bias supply must provide V CC (PWM controller)
and V DD (gate-drive power), so the maximum current
drawn is:
I BIAS = I CC + f (Q G1 + Q G2 ) = 10mA to 40mA (typ)
where I CC is 800µA (typ), f is the switching frequency,
and Q
G1
and Q
G2
are the MOSFET data sheet total
gate-charge specification limits at V GS = 5V.
V+ and V
DD
can be tied together if the input power
source is a fixed 4.5V to 5.5V supply. If the 5V bias
supply is powered up prior to the battery supply, the
enable signal (SKP/SDN going from low to high or
open) must be delayed until the battery voltage is pre-
sent to ensure startup.
Free-Running, Constant On-Time PWM
Controller with Input Feed-Forward
The Quick-PWM control architecture is a pseudofixed-
frequency, constant-on-time current-mode type with
voltage feed-forward (Figure 2). This architecture relies
on the output filter capacitor’s ESR to act as the cur-
rent-sense resistor, so the output ripple voltage pro-
vides the PWM ramp signal. The control algorithm is
simple: the high-side switch on-time is determined sole-
ly by a one-shot whose period is inversely proportional
to input voltage and directly proportional to output volt-
age. Another one-shot sets a minimum off-time (400ns
typ). The on-time one-shot is triggered if the error com-
parator is low, the low-side switch current is below the
current-limit threshold, and the minimum off-time one-
shot has timed out.
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 con-
stant 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-cur-
rent operating point remains relatively constant, resulting
in easy design methodology and predictable output
voltage ripple.
On-Time = K (V
OUT
+ 0.075V) / V
IN
where K is set by the TON pin-strap connection and
0.075V is an approximation to accommodate the expect-
ed drop across the low-side MOSFET switch (Table 2).
The on-time one-shot has good accuracy at the operating
points specified in the Electrical Characteristics table
(±10% at 200kHz and 300kHz, ±12% at 550kHz and
1000kHz). On-times at operating points far removed from
the conditions specified in the Electrical Characteristics
table can vary over a wider range. For example, the
1000kHz setting will typically run about 10% slower with
inputs much greater than +5V due to the very short on-
times required.
On-times translate only roughly to switching frequencies.
The on-times guaranteed in the Electrical Character-
istics table are influenced by switching delays in the
Table 1. Component Suppliers
MANUFACTURER USA PHONE FACTORY FAX
Central Semiconductor 516-435-1110 516-435-1824
Dale-Vishay 402-564-3131 402-563-6418
Fairchild 408-721-2181 408-721-1635
International Rectifier 310-322-3331 310-322-3332
Kemet 408-986-0424 408-986-1442
Motorola 602-303-5454 602-994-6430
Nihon 847-843-7500 847-843-2798
Panasonic 714-373-7939 714-373-7183
Taiyo Yuden 408-573-4150 408-573-4159
TDK 847-390-4373 847-390-4428
Toko 800-745-8656 408-943-9790
Sanyo 619-661-6835 619-661-1055
SGS-Thomson 617-259-0300 617-259-9442
Sumida 708-956-0666 708-956-0702
Zetex 516-543-7100 516-864-7630