LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 9

• Manufacturer Part Number: LTC3805-5
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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OPERATION
Start-Up/Shutdown
The LTC3805-5 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the V
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 insuffi cient voltage on the V
at the V
able LTC3805-5 operation. After operation is enabled, the
voltage on the V
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
disabled when the voltage falls below V
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.
CC
pin must initially exceed V
CC
pin may fall as low as V
RUNON
TURNON
CC
Figure 1. Start-Up/Shutdown State Diagram
RUNOFF
pin. The voltage
SHUTDOWN
LTC3805-5
V
= 1.207V and
TURNOFF
SSFLT
= 4.5V to en-
< 0.7V
= 1.170V.
AND V
= 4V
V
V
V
CC
RUN
RUN
CC
LTC3805-5
TIMEOUT
CC
< V
FAULT
> V
< V
> V
TURNOFF
RUNON
RUNOFF
TURNON
Setting the Oscillator Frequency
Connect a frequency set resistor R
ground to set the oscillator frequency over a range from
70kHz to 700kHz. The oscillator frequency is calculated
from:
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 pe-
riod, i.e., the GATE pin going high. The pulse width of the
external clock is quite fl exible. The clock must stay high
only for about 200ns to trigger the start of a new switching
period. Conversely, the pulse width can be increased to a
duty cycle not greater than 55%.
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
output current increases, so does the voltage on the I
pin and so does the peak MOSFET current.
TH
f
pin operating through the current comparator. As the
OSC
V
OC
> 100mV
LTC3805-5
=
ENABLED
R
FS
24 10
38055 F01
−
•
1500
9
LTC3805-5
FS
from the FS pin to
38055fa
9
TH