MAX1717 Maxim, MAX1717 Datasheet - Page 14

• Manufacturer Part Number: MAX1717
• Description: Dynamically Adjustable / Synchronous Step-Down Controller for Notebook CPUs
• Manufacturer: Maxim

Table 2. Component Suppliers
Dynamically Adjustable, Synchronous
Step-Down Controller for Notebook CPUs
The MAX1717 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
troller) and V
current drawn is:
where I
and Q
gate-charge specification limits at V
V+ and V
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.
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 current-
sense resistor, so the output ripple voltage provides the
PWM ramp signal. The control algorithm is simple: the
high-side switch on-time is determined solely by a one-
shot whose period is inversely proportional to input volt-
age and directly proportional to output voltage. Another
14
Coilcraft
Dale-Vishay
International Rectifier
Kemet
Panasonic
Sumida
Taiyo Yuden
I
MANUFACTURER
BIAS
______________________________________________________________________________________
Free-Running, Constant On-Time PWM
G1
CC
= I
DD
Controller with Input Feed-Forward
and Q
CC
is 700µA (typ), f is the switching frequency,
DD
can be tied together if the input power
+ f (Q
+5V Bias Supply (V
G2
(gate-drive power), so the maximum
G1
are the MOSFET data sheet total
Detailed Description
+ Q
847-639-6400
402-564-3131
310-322-3331
408-986-0424
714-373-7939
847-956-0666
408-573-4150
USA PHONE
G2
) = 10mA to 40mA (typ)
GS
= 5V.
[1] 847-639-1469
[1] 402-563-6418
[1] 310-322-3332
[1] 408-986-1442
[1] 714-373-7183
[81] 3-3607-5144
[1] 408-573-4159
CC
[Country Code]
CC
FACTORY FAX
and V
(PWM con-
DD
)
Table 3. Approximate K-Factors Errors
one-shot sets a minimum off-time (400ns typ). The on-
time one-shot is triggered if the error comparator is low,
the low-side switch current is below the current-limit
threshold, and the minimum off-time one-shot has timed
out.
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.
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 3).
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 wide 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
SETTING
(kHz)
1000
TON
200
300
550
FACTOR
On-Time = K (V
(µs)
3.3
1.8
1.0
K
5
APPROXIMATE
ERROR (%)
K-FACTOR
±12.5
±12.5
On-Time One-Shot (TON)
±10
±10
OUT
+ 0.075V) / V
V
MIN RECOMMENDED
BATT
AT V
(V)
2.1
2.3
3.2
4.5
IN
OUT
= 1.6V