MAX8643 Maxim Integrated Products, MAX8643 Datasheet - Page 10

• Manufacturer Part Number: MAX8643
• Description: 2MHz Step-Down Regulator
• Manufacturer: Maxim Integrated Products

www.DataSheet4U.com
Figure 2. Typical Soft-Start Implementation with External
Reference
3A, 2MHz Step-Down Regulator
with Integrated Switches
The internal, high-side MOSFET has a typical 5.5A
peak current-limit threshold. When current flowing out
of LX exceeds this limit, the high-side MOSFET turns off
and the synchronous rectifier turns on. The synchro-
nous rectifier remains on until the inductor current falls
below the low-side current limit. This lowers the duty
cycle and causes the output voltage to droop until the
current limit is no longer exceeded. The MAX8643 uses
a hiccup mode to prevent overheating during short-cir-
cuit output conditions.
During current limit if V
stays below this level for 12µs or more, the part enters
hiccup mode. The high-side MOSFET and the synchro-
nous rectifier are turned off and both COMP and REFIN
are internally pulled low. If REFIN and SS are connect-
ed together, then both are pulled low. The part remains
in this state for 1024 clock cycles and then attempts to
restart for 128 clock cycles. If the fault-causing current
limit has cleared, the part resumes normal operation.
Otherwise, the part reenters hiccup mode again.
The MAX8643 utilizes an adjustable soft-start function
to limit inrush current during startup. An 8µA (typ) cur-
rent source charges an external capacitor connected to
SS. The soft-start time is adjusted by the value of the
external capacitor from SS to GND. The required
capacitance value is determined as:
where t
MAX8643 also features an external reference input
(REFIN). The IC regulates FB to the voltage applied to
REFIN. The internal soft-start is not available when
using an external reference. A method of soft-start
when using an external reference is shown in Figure 2.
Connect REFIN to SS to use the internal 0.6V reference.
10
______________________________________________________________________________________
SS
R1
is the required soft-start time in seconds. The
R2
C
=
C
8
FB
μ
A t
0 6
Soft-Start and REFIN
.
drops below 420mV and
×
V
REFIN
SS
MAX8643
Current Limit
Table 1. CTL1 and CTL2 Output Voltage
Selection
The UVLO circuitry inhibits switching when V
below 2V (typ). Once V
clears and the soft-start function activates. A 100mV
hysteresis is built in for glitch immunity.
The gate-drive voltage for the high-side, n-channel
switch is generated by a flying-capacitor boost circuit.
The capacitor between BST and LX is charged from the
V
low-side MOSFET is switched off, the voltage of the
capacitor is stacked above LX to provide the necessary
turn-on voltage for the high-side internal MOSFET.
The switching frequency is resistor programmable from
500kHz to 2MHz. Set the switching frequency of the IC
with a resistor (R
R
where f
PWRGD is an open-drain output that goes high imped-
ance when V
low when V
48 clock cycles. PWRGD is low during shutdown.
As shown in Table 1, the output voltage is pin program-
mable by the logic states of CTL1 and CTL2. CTL1 and
CTL2 are tri-level inputs: V
IN
FREQ
Unconnected
Unconnected
Unconnected
supply while the low-side MOSFET is on. When the
CTL1
is calculated as:
S
GND
GND
GND
V
V
V
DD
DD
DD
is the desired switching frequency in Hz.
R
FB
FREQ
Programming the Output Voltage
FB
is below 90% of its regulation for at least
Undervoltage Lockout (UVLO)
is above 0.9 x V
FREQ
Power-Good Output (PWRGD)
=
0 95
50
.
) connected from FREQ to GND.
Unconnected
Unconnected
Unconnected
Frequency Select (FREQ)
k
DD
Ω
μ
CTL2
s
GND
GND
GND
V
V
V
DD
rises above 2V (typ), UVLO
DD
DD
DD
×
(
, unconnected, and GND.
f
1
S
−
REFIN
0 05
.
(CTL1, CTL2)
μ
. PWRGD pulls
s
)
V
OUT
0.6
0.7
0.8
1.0
1.2
1.5
1.8
2.0
2.5
(V)
DD
BST
is