LT3599(RevD) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT3599 Datasheet PDF : 26 Pages
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LT3599
Applications Information
Switching Frequency Synchronization
The nominal operating frequency of the LT3599 is pro-
grammed using a resistor from the RT pin to ground
and can be controlled over a 200kHz to 2.1MHz range. In
addition, the internal oscillator can be synchronized to an
external clock applied to the SYNC pin. The synchronizing
clock signal input to the LT3599 must have a frequency
between 240kHz and 1.5MHz, a pulse on-time of at least
150ns, a pulse off-time of at least 300ns, a low state below
0.8V and a high state above 1.7V. Synchronization signals
outside of these parameters will cause erratic switching
behavior. For proper operation, an RT resistor should be
chosen to program a switching frequency 20% slower
than the SYNC pulse frequency. Synchronization occurs
at a fixed delay after the rising edge of SYNC.
The SYNC pin should be grounded if the clock synchroniza-
tion feature is not used. When the SYNC pin is grounded,
the internal oscillator generates switching frequency to
the converter.
Shutdown and Programming Undervoltage Lockout
The LT3599 has an accurate 1.4V shutdown threshold
at the SHDN/UVLO pin. This threshold can be used in
conjunction with a resistor divider from the system input
supply to define an accurate undervoltage lockout (UVLO)
threshold for the system (Figure 10). A pin current hys-
teresis allows programming of the hysteresis voltage for
this UVLO threshold. Just before the part turns on, 4µA
10000
1000
flows into the SHDN/UVLO pin. After the part turns on, 0µA
flows from the SHDN/UVLO pin. Calculation of the on/off
thresholds for a system input supply using the LT3599
SHDN/UVLO pin can be made as follows :
VS(OFF
)
=
1.4
1+
R1
R2
VS(ON) = VS(OFF) + (4µA • R1)
A simple open drain transistor can be added to the resistor
divider network at the SHDN/UVLO pin to independently
control the turn off of the LT3599.
With the SHDN/UVLO pin connected directly to the VIN pin,
an internal undervoltage lockout threshold of approximately
2.7V exists for the VIN pin. This prevents the converter
from operating in an erratic mode when supply voltage is
too low. The LT3599 provides a soft-start function when
recovering from such faults as SHDN < 1.4V and/or VIN
< 2.7V. See “Soft-Start” in the Applications Information
section for details.
Soft-Start and Switching Frequency Foldback
To limit inrush current and output voltage overshoot dur-
ing start-up/recovery from a fault condition, the LT3599
provides a soft-start pin, SS. The SS pin is used to program
switch current ramp-up timing using a capacitor to ground.
The LT3599 monitors system parameters for the following
faults: VIN < 2.7V or SHDN < 1.4. On detection of any of
these faults, the LT3599 stops switching immediately and
VS
LT3599
R1
11 SHDN/UVLO
–
ON
OFF
R2
1.4V +
4µA
100
10
100
RT (k)
1000
3599 F10
Figure 9. Switching Frequency
3599 F11
Figure 10. Programming Undervoltage
Lockout (UVLO) with Hysteresis
3599fd
15
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