Datasheet

LTC3805
9
3805fg
Figure 1. Start-Up/Shutdown State Diagram
LTC3805
SHUTDOWN
LTC3805
FAULT
TIMEOUT
3805 F01
V
RUN
> V
RUNON
AND V
CC
> V
TURNON
V
RUN
< V
RUNOFF
V
CC
< V
TURNOFF
V
SSFLT
< 0.7V V
OC
> 100mV
LTC3805
ENABLED
operation
Start-Up/Shutdown
The LTC3805 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the V
CC
supply pin voltage, and a precision-threshold RUN pin. The
voltage on both pins must exceed the appropriate thresh-
old before operation is enabled. The LTC3805 transitions
into and out of shutdown according to the state diagram
shown in Figure 1. Operation in fault timeout is discussed
in a subsequent section. During shutdown the LTC3805
draws only a small 40µA current.
The undervoltage lockout (UVLO) mechanism prevents
the LTC3805 from trying to drive the external MOSFET
gate with insufficient voltage on the V
CC
pin. The voltage
at the V
CC
pin must initially exceed V
TURNON
= 8.4V to
enable LTC3805 operation. After operation is enabled, the
voltage on the V
CC
pin may fall as low as V
TURNOFF
= 4V
before undervoltage lockout disables the LTC3805. This
wide UVLO hysteresis range supports the use of trickle
charger powering schemes. See the Applications Informa-
tion section for more detail.
The RUN pin is connected to the input voltage using a
voltage divider. Converter operation is enabled when the
voltage on the RUN pin exceeds V
RUNON
= 1.207V and
disabled when the voltage falls below V
RUNOFF
= 1.170V.
Additional hysteresis is added by a 5µA current source
acting on the voltage divider’s Thevenin resistance. Setting
the input voltage range and hysteresis is further discussed
in the Applications Information section.
Setting the Oscillator Frequency
Connect a frequency set resistor R
FS
from the FS pin to
ground to set the oscillator frequency over a range from
70kHz to 700kHz. The oscillator frequency is calculated
from:
f
OSC
=
24 • 10
9
R
FS
− 1500
The oscillator may be synchronized to an external clock
using the SYNC input. The rising edge of the external clock
on the SYNC pin triggers the beginning of a switching
period, i.e., the GATE pin going high. The pulse width of
the external clock is quite flexible.
Overcurrent Protection
With the OC pin connected to the external MOSFET’s current
sense resistor, the converter is protected in the event of
an overload or short-circuit on the output. During normal
operation the peak value of current in the external MOS-
FET, as measured by the current sense resistor (plus any
adjustment for slope compensation), is set by the voltage
on the I
TH
pin operating through the current comparator.
As the output current increases, so does the voltage on
the I
TH
pin and so does the peak MOSFET current.