MAX8734AEEI+G104 Maxim Integrated, MAX8734AEEI+G104 Datasheet - Page 18

• Manufacturer Part Number: MAX8734AEEI+G104
• Description: Current & Power Monitors & Regulators High-Efficiency Quad-Output Main Power-Supply Controllers for Notebook Computers
• Manufacturer: Maxim Integrated

Specifications of MAX8734AEEI+G104

• Product: Power Monitors
• Supply Voltage - Max: 24 V
• Supply Voltage - Min: 4.5 V
• Operating Temperature Range: - 40 C to + 85 C
• Accuracy: 1.50%
• Input Voltage Range: 4.5 V to 24 V
• Supply Current: 50 uA

High-Efficiency, Quad-Output, Main Power-
Supply Controllers for Notebook Computers
These internal blocks are not powered directly from the
battery. Instead, the 5V (LDO5) linear regulator steps
down the battery voltage to supply both internal circuit-
ry and the gate drivers. The synchronous-switch gate
drivers are directly powered from LDO5, while the high-
side switch gate drivers are indirectly powered from
LDO5 through an external diode-capacitor boost cir-
cuit. An automatic bootstrap circuit turns off the 5V lin-
ear regulator and powers the device from OUT5 when
OUT5 is above 4.56V.
The Quick-PWM control architecture is a pseudo-fixed-
frequency, constant on-time, current-mode type with
voltage feed-forward. The Quick-PWM control architec-
ture relies on the output ripple voltage to provide the
PWM ramp signal; thus, the output filter capacitor’s
ESR acts as a current-feedback resistor. The high-side
switch on-time is determined by a one-shot whose peri-
od is inversely proportional to input voltage and directly
proportional to output voltage. Another one-shot sets a
minimum off-time (300ns typ). The on-time, one-shot
triggers when the following conditions are met: the error
comparator is low, the synchronous rectifier current is
below the current-limit threshold, and the minimum off-
time one-shot has timed out.
Each PWM core includes a one-shot that sets the high-
side switch on-time for each controller. Each 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 benefit
of a constant switching frequency is the frequency can
be selected to avoid noise-sensitive frequency regions:
Table 2. Approximate K-Factor Errors
18
MAX8732A/MAX8734A (t
MAX8732A/MAX8734A (t
MAX8733A/MAX8734A (t
MAX8733A/MAX8734A (t
______________________________________________________________________________________
Free-Running, Constant On-Time PWM
Controller with Input Feed-Forward
t
ON
SMPS
=
K V
ON
ON
ON
ON
(
On-Time, One-Shot (t
= V
= V
= GND), 5V
= GND), 3.3V
OUT
CC
CC
V
+
), 5V
), 3.3V
+
0 075
.
V
)
SWITCHING FREQUENCY
ON
)
(kHz)
200
300
400
500
See Table 2 for approximate K-factors. The constant
0.075V is an approximation to account for the expected
drop across the synchronous-rectifier switch. Switching
frequency increases as a function of load current due
to the increasing drop across the synchronous rectifier,
which causes a faster inductor-current discharge ramp.
On-times translate only roughly to switching frequen-
cies. The on-times guaranteed in the Electrical
Characteristics are influenced by switching delays in
the external high-side power MOSFET. Also, the dead-
time effect increases the effective on-time, reducing the
switching frequency. It occurs only in PWM mode (SKIP
= V
when the inductor current reverses at light or negative
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 DH-
rising dead time.
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; V
the sum of the parasitic voltage drops in the charging
path, including high-side switch, inductor, and PC
board resistances, and t
the MAX8732A/MAX8733A/MAX8734A.
In Idle Mode (SKIP = GND), an inherent automatic
switchover to PFM takes place at light loads. This
switchover is affected 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-
CC
) and during dynamic output voltage transitions
Automatic Pulse-Skipping Switchover
DROP1
K-FACTOR (µs)
is the sum of the parasitic voltage drops
f
=
5.0
3.3
2.5
2.0
t
ON
V
OUT
(
ON
V
+ +
is the on-time calculated by
+
V
V
DROP
DROP
FACTOR ERROR (%)
APPROXIMATE K-
1
2
)
(Idle Mode)
±10
±10
±10
±10
DROP2
is