lt1339 Linear Technology Corporation, lt1339 Datasheet - Page 11

• Manufacturer Part Number: lt1339
• Description: High Power Synchronous Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
impedance of the I
quency is typically set 1 to 2 orders of magnitude below the
oscillator frequency and follows the relation:
The average current limit function can be disabled by
shorting the I
Soft Start Programming
The current control pin (V
to zero at voltages less than a transistor V
current limit at V
regulation range for average load current. An internal
voltage clamp forces the V
the SS pin voltage. This 100mV “dead zone” assures 0%
duty cycle operation at the start of the soft start cycle, or
when the soft start pin is pulled to ground. Given the
typical soft start current of 8 A and a soft start timing
capacitor C
average current will be:
Boost Supply
The V
tor. This supply provides gate drive to the topside switch
FET. The bootstrap capacitor is charged from 12V
a diode when the switch node is pulled low.
The diode reverse breakdown voltage must be greater than
V
times greater than the total input capacitance of the
topside FET. A capacitor in the range of 0.1 F to 1 F is
generally adequate for most applications.
Shutdown Function — Input Undervoltage Detect and
Threshold Hysteresis
The LT1339 RUN/SHDN pin uses a bandgap generated
reference threshold of about 1.25V. This precision thresh-
old allows use of the RUN/SHDN pin for both logic-level
shutdown applications and analog monitoring applica-
tions such as power supply sequencing.
Because an LT1339 controlled converter is a power
transfer device, a voltage that is lower than expected on
the input supply could require currents that exceed the
IN
f
t
–3dB
SS
+ 12V
BOOST
= (1.5)(10
= (3.2)(10
IN
. The bootstrap capacitor should be at least 100
SS
supply is bootstrapped via an external capaci-
AVG
, the start-up delay time to full available
C
5
)(C
pin directly to SGND.
= V
– 6
U
AVG
SS
)/C
BE
)
AVG
+ 1.8V. This generates a 1.8V full
INFORMATION
pin. The integrator corner fre-
C
U
) limits sensed inductor current
C
pin to a V
W
BE
BE
– 100mV above
, to full average
U
IN
through
sourcing capabilities of that supply, causing the system to
lock up in an undervoltage state. Input supply start-up
protection can be achieved by enabling the RUN/SHDN
pin using a resistor divider from the input supply to
ground. Setting the divider output to 1.25V when that
supply is almost fully enabled prevents the LT1339 regu-
lator from drawing large currents until the input supply is
able to provide the required power.
If additional hysteresis is desired for the enable function,
an external feedback resistor can be used from the LT1339
regulator output. If connection to the regulator output is
not desired, the 5V
Figure 3 shows a resistor connection on a 48V to 5V
converter that yields a 40V V
regulator enable and also provides about 10% input
referred hysteresis.
The shutdown function can be disabled by connecting the
RUN/SHDN pin to the 12V
clamped to 2.5V through a 20k series input resistance and
will therefore draw about 0.5mA when tied directly to 12V.
This additional current can be minimized by making the
connection through an external resistor (100k is typically
used).
Operation with Split Supplies and Supply Sequencing
N-channel power MOSFETs can parasitically turn them-
selves on due to leakage currents or capacitive coupling
onto the MOSFET gate. In shutdown, this is prevented by
active pull-down clamps on the TG and BG driver outputs
of the LT1339. These clamps are active when 12V
The 12V
from the converter input supply; however, these supplies
can be independent. If these supplies are independent and
Figure 3. Input Supply Sequencing Programming
IN
power supply for the LT1339 is usually derived
48V
V
IN
300k
10k
390k
REF
OPTION 1
OPTION 2
internal supply pin can be used.
IN
13
2
rail. This pin is internally
IN
5V
RUN/SHDN
REF
start-up threshold for
LT1339
V
5V
OUT
1339 • F03
LT1339
sn1339 1339fas
IN
11
> 0.7V.