Datasheet

LT3599
15
3599fe
Switching Frequency Synchronization
The nominal operating frequency of the LT3599 is
programmed 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 2MHz, a pulse on-time of at least
50ns, a pulse off-time of at least 300ns, a low state below
0.8V and a high state above 2V. Synchronization signals
outside of these parameters will cause erratic switching
behavior. For proper operation, an R
T
resistor should be
chosen to program a switching frequency 20% slower
than the SYNC pulse frequency. Synchronization occurs
at a fi xed 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 defi ne an accurate undervoltage lockout
(UVLO) threshold for the system (Figure 10). A pin current
hysteresis allows programming of the hysteresis voltage
for this UVLO threshold. Just before the part turns on,
APPLICATIONS INFORMATION
4μA fl ows into the SHDN/UVLO pin. After the part turns
on, 0μA fl ows 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 :
V
S(OFF)
=1.4 1+
R1
R2
⎛
⎝
⎜
⎞
⎠
⎟
V
S(ON)
= V
S(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 V
IN
pin,
an internal undervoltage lockout threshold of approximately
2.7V exists for the V
IN
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 V
IN
< 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
during 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: V
IN
< 2.7V or SHDN < 1.4. On detection of any of
these faults, the LT3599 stops switching immediately and
Figure 10. Programming Undervoltage
Lockout (UVLO) with Hysteresis
Figure 9. Switching Frequency
3599 F11
V
S
R1
11
LT3599
R2
1.4V
4μA
SHDN/UVLO
–
+
ON
OFF
RT (k)
SWITCHING FREQUENCY (kHz)
3599 F10
10000
1000
100
10 100 1000