Datasheet

LTC3805-5
9
38055fe
Figure 1. Start-Up/Shutdown State Diagram
LTC3805-5
SHUTDOWN
LTC3805-5
FAULT
TIMEOUT
38055 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
LTC3805-5
ENABLED
OPERATION
Start-Up/Shutdown
The LTC3805-5 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 threshold
before operation is enabled. The LTC3805-5 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 LTC3805-5
draws only a small 40µA current.
The undervoltage lockout (UVLO) mechanism prevents
the LTC3805-5 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
= 4.5V to en-
able LTC3805-5 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 LTC3805-5. See
the Applications Information 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 MOSFET,
as measured by the current sense resistor (plus any adjust-
ment 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.