MAX1717 Maxim, MAX1717 Datasheet - Page 17

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

where K is the on-time scale factor (Table 3). The load-
current level at which PFM/PWM crossover occurs,
I
rent, which is a function of the inductor value (Figure 4).
For example, in the standard application circuit this
becomes:
Figure 3. Voltage-Positioned Circuit
LOAD(SKIP)
I
LOAD SKIP
(
, is equal to 1/2 the peak-to-peak ripple cur-
TO V
3 3
.
CC
2 1
)
µ
Step-Down Controller for Notebook CPUs
⋅
s
HIGH/LOW
≈
⋅
______________________________________________________________________________________
µ
1 6
BIAS SUPPLY
H
.
K
V
2
⋅
CONTROL
V
Dynamically Adjustable, Synchronous
⋅
BATT
ON/OFF
+5V
V
⋅
12
OUT
100k
L
V
12
−
120k
R7
V
1 6
470pF
⋅
1µF
C4
.
C3
V
V
1µF
C5
21
20
19
18
17
BATT
16
1
2
3
8
9
6
=
V+
SKP/SDN
TIME
D0
D1
D2
D3
D4
TON
REF
CC
A/B
V
2 3
V
CC
BATT
.
7
−
A
MAX1717
TO V
20Ω
ILIM
R1
V
10
CC
OUT
VGATE
GNDS
V
GND
BST
FBS
DD
DH
DL
LX
FB
15
23
14
13
11
12
22
24
4
5
+5V
D2
CMPSH-3
C7
0.1µF
R2
100k
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used.
The switching waveforms may appear noisy and asyn-
chronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM
noise vs. light-load efficiency are made by varying the
inductor value. Generally, low inductor values produce
a broader efficiency vs. load curve, while higher values
result in higher full-load efficiency (assuming that the
coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values
include larger physical size and degraded load-tran-
sient response (especially at low input voltage levels).
1µF
C6
POWER-GOOD
INDICATOR
Q1
Q2
C1
1µH
L1
D1
D1 = INTL RECT 10MQ040N.
FOR OTHER COMPONENTS,
SEE TABLE 1 VALUES.
0.005Ω
150k
R4
2k
R5
R6
TO V
C2
REF
V
A/B = LOW = 1.60V
A/B = HIGH = 1.35V
OUT
17