MAX1715 Maxim, MAX1715 Datasheet - Page 14

• Manufacturer Part Number: MAX1715
• Description: Ultra-High Efficiency / Dual Step-Down Controller for Notebook Computers
• Manufacturer: Maxim

Ultra-High Efficiency, Dual Step-Down
Controller for Notebook Computers
nominal frequency setting (200kHz, 300kHz, 420kHz, or
540kHz), while the on-times for side 2 are set 15%
lower than nominal. This is done to prevent audio-fre-
quency “beating” between the two sides, which switch
asynchronously for each side:
where K is set by the TON pin-strap connection and
0.075V is an approximation to accommodate for the
expected drop across the low-side MOSFET switch.
One-shot timing error increases for the shorter on-time
settings due to fixed propagation delays; it is approxi-
mately ±12.5% at 540kHz and 420kHz nominal settings
and ±10% at the two slower settings. This translates to
reduced switching-frequency accuracy at higher fre-
quencies (Table 5). Switching frequency increases as a
function of load current due to the increasing drop
across the low-side MOSFET, which causes a faster
inductor-current discharge ramp. The on-times guaran-
teed in the Electrical Characteristics are influenced by
switching delays in the external high-side power MOS-
FET.
Two external factors that influence switching-frequency
accuracy are resistive drops in the two conduction
loops (including inductor and PC board resistance) and
the dead-time effect. These effects are the largest con-
tributors to the change of frequency with changing load
current. The dead-time effect increases the effective
on-time, reducing the switching frequency as one or
both dead times. It occurs only in PWM mode (SKIP =
high) when the inductor current reverses at light or neg-
ative load currents. With reversed inductor current, the
inductor’s EMF causes LX to go high earlier than nor-
mal, extending the on-time by a period equal to the
low-to-high dead time.
14
Table 4. Frequency Selection Guidelines
FREQUENCY
NOMINAL
(kHz)
______________________________________________________________________________________
200
300
420
540
On-Time = K (V
4-cell Li+ notebook
4-cell Li+ notebook
3-cell Li+ notebook
+5V input
APPLICATION
TYPICAL
OUT
+ 0.075V) / V
Use for absolute best
efficiency.
Considered mainstream
by current standards.
Useful in 3-cell systems
for lighter loads than the
CPU core or where size is
key.
Good operating point for
compound buck designs
or desktop circuits.
COMMENTS
IN
For loads above the critical conduction point, the actual
switching frequency is:
where V
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; VDROP2
is the sum of the resistances in the charging path; and
t
In skip mode (SKIP low), an inherent automatic
switchover to PFM takes place at light loads. This
switchover is effected by a comparator that truncates
the low-side switch on-time at the inductor current’s
zero crossing. This mechanism causes the threshold
between pulse-skipping PFM and nonskipping PWM
operation to coincide with the boundary between con-
tinuous and discontinuous inductor-current operation
(also known as the “critical conduction” point). For a
battery range of 7V to 24V, this threshold is relatively
constant, with only a minor dependence on battery volt-
age.
where K is the on-time scale factor (Table 5). 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 with
V
5), switchover to pulse-skipping operation occurs at
I
occurs at an even lower value if a swinging (soft-satura-
tion) inductor is used.
The switching waveforms may appear noisy and asyn-
chronous when light loading causes pulse-skipping
Table 5. Approximate K-Factor Errors
ON
LOAD(SKIP)
LOAD
SETTING
OUT1
OPEN
GND
TON
V
REF
is the on-time calculated by the MAX1715.
CC
= 0.7A or about 1/6 full load. The crossover point
Automatic Pulse-Skipping Switchover
= 2.5V, V
DROP
I
LOAD(SKIP)
ERROR (%)
K-FACTOR
, is equal to 1/2 the peak-to-peak ripple cur-
APPROX
±12.5
±12.5
1 is the sum of the parasitic voltage drops
±10
±10
f
IN
=
= 15V, and K = 2.96µs (see Table
t
≈
ON IN
V
K V
OUT
AT V
(
= 2V (V)
MIN V
⋅
V
2L
2.6
2.9
3.2
3.6
OUT_
+
+
OUT
V
IN
V
DROP
DROP



V - V
IN
SIDE 1 K
FACTOR
1
2
V
4.24
2.96
2.08
1.63
(µs)
)
IN
OUT



SIDE 2 K
FACTOR
5.81
4.03
2.81
2.18
(µs)