Rainbow Electronics MAX5083 User Manual
Page 9
MAX5082/MAX5083
1.5A, 40V, MAXPower Step-Down
DC-DC Converters
_______________________________________________________________________________________
9
Soft-Start and Reference (SS)
SS is the 1.23V reference bypass connection for the
MAX5082/MAX5083 and also controls the soft-start
period. At startup, after V
IN
is applied and the internal
and external UVLO thresholds are reached, the device
enters soft-start. During soft-start, 15µA is sourced into
the capacitor (C
SS
) connected from SS to SGND caus-
ing the reference voltage to ramp up slowly. When V
SS
reaches 1.23V the output becomes fully active. Set the
soft-start time (t
SS
) using the following equation:
where t
SS
is in seconds and C
SS
is in Farads.
Internal Charge Pump (MAX5082)
The MAX5082 features an internal charge pump to
enhance the turn-on of the internal MOSFET, allowing
for operation with input voltages down to 4.5V. Connect
a flying capacitor (C
F
) between C+ and C-, a boost
diode from C+ to BST, as well as a bootstrap capacitor
(C
BST
) between BST and LX to provide the gate-drive
voltage for the high-side n-channel DMOS switch.
During the on-time, the flying capacitor is charged to
V
DVREG
. During the off-time, the positive terminal of the
flying capacitor (C+) is pumped to two times V
DVREG
and charge is dumped onto C
BST
to provide twice the
regulator voltage across the high-side DMOS driver.
Use a ceramic capacitor of at least 0.1µF for C
BST
and
C
F
located as close to the device as possible.
For applications that do not require a 4.5V minimum
input, use the MAX5083. In this device, the charge
t
V
C
A
SS
SS
=
×
1 23
15
.
µ
MAX5083
1.23V
1.23V
ON/OFF
LDO
EN
IN
REG
SS
FB
THERMAL
SHDN
REF
IN
0.3V
CLK
ILIM
1.23V
REGOK
EN
V
REF
V
REF
I
SS
>1.23V ON
<1.11V OFF
COMP
SYNC
E/A
SSA
CPWM
RAMP
LOGIC
EN
OSC
BOOTSTRAP
CONTROL
PCLK
BST
IN
SGND
LX
DVREG
PGND
SCLK
HIGH-SIDE
CURRENT
SENSE
REF_PFM
PFM
REF_ILIM
ILIM
OVERL
CLK
ILIM
OVERLOAD
MANAGEMENT
Figure 2. MAX5083 Simplified Block Diagram