Rainbow Electronics MAX5081 - Manual (Page 10)

MAX5080/MAX5081
1A, 40V, MAXPower Step-Down
DC-DC Converters
10 ______________________________________________________________________________________
Soft-Start and Reference (SS)
SS is the 1.23V reference bypass connection for the
MAX5080/MAX5081 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 following equation:
where t SS is in seconds and C SS is in Farads.
Internal Charge Pump (MAX5080)
The MAX5080 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.
t
V C
A
SS
SS =
× 1 23
15
.
µ
MAX5081
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. MAX5081 Simplified Block Diagram