MAX8717ETI+ Maxim Integrated Products, MAX8717ETI+ Datasheet - Page 17

• Manufacturer Part Number: MAX8717ETI+
• Description: IC CNTRLR PWR SUP 28-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8717ETI+

• Applications: Controller, Notebook Computers
• Voltage - Input: 4 ~ 26 V
• Number Of Outputs: 2
• Voltage - Output: 3.3V, 5V, 1 ~ 5.5 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN Exposed Pad
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

the switching frequency remains constant and the
no-load supply current is typically between 8mA and
20mA per phase, depending on external MOSFETs and
switching frequency.
The MAX8716/MAX8717/MAX8756/MAX8757 include
SKIP_ inputs that enable the corresponding outputs to
operate in discontinuous mode. Connect SKIP_ to GND
or REF as shown in Table 4 to enable or disable the
zero-crossing comparators of either controller. When
the zero-crossing comparator is enabled, the controller
forces DL_ low when the current-sense inputs detect
zero inductor current. This keeps the inductor from dis-
charging the output capacitors and forces the con-
troller to skip pulses under light-load conditions to
avoid overcharging the output. During skip mode, the
V
improved. During low-noise skip mode, the no-load rip-
ple amplitude is two times smaller and the no-load
switching frequency is four times higher, although the
light-load efficiency is somewhat lower.
When pulse-skipping mode is enabled, the on-time of
the step-down controller terminates when the output
voltage exceeds the feedback threshold and when the
current-sense voltage exceeds the Idle Mode current-
sense threshold. Under light-load conditions, the on-
time duration depends solely on the Idle Mode
current-sense threshold (SKIP_ = GND), which is 20%
of the full-load current-limit threshold set by ILIM_, or
the low-noise current-sense threshold (SKIP_ = REF),
which is 10% for the MAX8716/MAX8717/MAX8757 and
5% for the MAX8756 of the full-load current-limit thresh-
old set by ILIM_. This forces the controller to source a
minimum amount of power with each cycle. To avoid
overcharging the output, another on-time cannot begin
until output voltage drops below the feedback thresh-
old. Since the zero-crossing comparator prevents the
switching regulator from sinking current, the controller
must skip pulses. Therefore, the controller regulates the
valley of the output ripple under light-load conditions.
Table 3. FSEL Configuration Table
DD
Interleaved High-Efficiency, Dual Power-Supply
current consumption is reduced and efficiency is
FSEL
GND
V
REF
Light-Load Operation Control ( SKIP_ )
CC
Idle Mode Current-Sense Threshold
______________________________________________________________________________________
MAX8757 (kHz)
MAX8717/
500
300
200
Controllers for Notebook Computers
MAX8756 (kHz)
400
300
250
In skip mode, an inherent automatic switchover to PFM
takes place at light loads (Figure 4). This switchover is
affected by a comparator that truncates the low-side
switch on-time at the inductor current’s zero crossing.
The zero-crossing comparator senses the inductor cur-
rent across CSH_ and CSL_. Once V
below the 3mV zero-crossing, current-sense threshold,
the comparator forces DL_ low (Figure 3). This mecha-
nism causes the threshold between pulse-skipping PFM
and nonskipping PWM operation to coincide with the
boundary between continuous and discontinuous induc-
tor-current operation (also known as the “critical conduc-
tion” point). The load-current level at which PFM/PWM
crossover occurs, I
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
inductance. Generally, low inductance produces 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).
Figure 4. Pulse-Skipping/Discontinuous Crossover Point
0
I
LOAD SKIP
ON-TIME
Automatic Pulse-Skipping Crossover
(
t
ON(SKIP)
LOAD(SKIP)
)
=
=
V
(
IN
V
V
OUT
x f
IN
OSC
2
, is determined by:
−
LV
TIME
V
IN OSC
OUT
ƒ
CSH
)
V
OUT
I
LOAD
- V
I
=
LOAD(SKIP)
CSL
I
LOAD(SKIP)
2
_ drops
17