MAX8543 MAXIM [Maxim Integrated Products], MAX8543 Datasheet - Page 17

• Manufacturer Part Number: MAX8543
• Description: Step-Down Controllers with Prebias Startup, Lossless Sensing, Synchronization, and OVP
• Manufacturer: MAXIM [Maxim Integrated Products]

Use very short, wide traces, about 10 to 20 squares
(50 mils to 100 mils wide if the MOSFET is 1in from the
device) for the gate drive. The dead time at the other
edge (DH turning off) also has an adaptive dead-time
circuit operating in a similar manner. For both edges,
there is an additional fixed dead time after the adaptive
dead time expires.
A flying capacitor boost circuit (Figure 3) generates the
gate-drive voltage for the high-side n-channel MOSFET.
The capacitor between BST and LX is charged from VL
up to V
low-side MOSFET is on. When the low-side MOSFET is
switched off, the stored voltage of the capacitor is
stacked above LX to provide the necessary turn-on
voltage (V
then closes an internal switch between BST and DH to
turn the high-side MOSFET on.
The MAX8543/MAX8544 current-sense circuit amplifies
the differential current-sense voltage (V
gain of the current-sense amplifier is determined by the
states of ILIM and ILIM1. This amplified current-sense
signal and the internal slope-compensation signal are
summed (V
parator’s inverting input. The PWM comparator shuts
off the high-side MOSFET when V
integrated feedback voltage (V
The differential current sense is also used to provide
peak inductor current limiting. This current limit is more
accurate than the valley current limit, which is measured
across the low-side MOSFET’s on-resistance.
Figure 3. The boost circuit provides voltage for the high-side
MOSFET gate drive.
VL
Lossless Sensing, Synchronization, and OVP
Step-Down Controllers with Prebias Startup,
minus the diode forward-voltage drop while the
GS
High-Side Gate-Drive Supply (BST)
SUM
) for the high-side MOSFET. The controller
MAX8543/
MAX8544
) together and fed into the PWM com-
______________________________________________________________________________________
IN
Current-Sense Amplifier
BST
DH
DL
LX
COMP
).
SUM
CS+
N
N
exceeds the
- V
CS-
). The
The MAX8543/MAX8544 use both valley foldback current
limiting and peak constant current limiting, simultaneously
(Figure 4). The valley foldback current limit is used to
reduce power dissipation of external components, mainly
inductor and power MOSFETs, and upstream power
source, when output is severely overloaded or short
circuited. Thus the circuit can withstand short-circuit
conditions indefinitely without causing overheating of any
component. The peak constant current limit sets the cur-
rent-limit point more accurately since it does not have to
suffer the wide variation of the low-side power MOSFET’s
on-resistance due to tolerance and temperature.
The valley current is sensed across the on-resistance of
the low-side MOSFET (V
limit trips when the sensed current exceeds the valley
current-limit threshold. The valley current limit recovers
when the sensed current drops below the valley current-
limit threshold (except when using the latch-off option
with the MAX8544).
Set the minimum valley current-limit threshold, when the
output voltage is at a nominal regulated value, higher
than the maximum peak current-limit setting. With this
method, the current-limit point accuracy is controlled by
the peak current limit and is not interfered with by the
wide variation of MOSFET on-resistance. See the Setting
the Current Limits section for how to set these limits.
The MAX8543 has a fixed valley current-limit threshold
and fixed foldback ratio. The MAX8544 can select
between an adjustable valley current-limit threshold
with adjustable foldback ratio and a fixed valley current
limit without foldback for latch-off. When latch-off is
used (MODE is connected to VL), set the current-limit
threshold by only one resistor from ILIM2 to GND and
make sure this threshold is higher than the maximum
output current required by at least a 20% margin. Cycle
EN or input power to reset the current-limit latch.
The peak current limit is used to sense the inductor
current, and is more accurate than the valley current limit
since it does not depend upon the on-resistance of the
low-side MOSFET. The peak current can be measured
across the resistance of the inductor for the highest
efficiency, or alternatively, a current-sense resistor can
be used for more accurate current sensing. The
MAX8543/MAX8544 have four selectable peak current-
limit thresholds that are selected using ILIM (MAX8543)
or ILIM1 (MAX8544). See Table 3 for the current-limit
settings.
For more information on the current limit, see the
Setting the Current Limits section.
PGND
Current-Limit Circuit
- V
LX
). The valley current
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