LT3501EFE Linear Technology, LT3501EFE Datasheet - Page 18

• Manufacturer Part Number: LT3501EFE
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,BIPOLAR,TSSOP,20PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LT3501
the output capacitor integrates this current, and that the
capacitor on the V
fi er output current, resulting in two poles in the loop. In
most cases a zero is required and comes from either the
output capacitor ESR or from a resistor in series with C
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. A phase lead
capacitor (C
the transient response.
Synchronization
The R
to an external clock source. Driving the R
with a clock source triggers the synchronization detection
circuitry. Once synchronization is detected, the rising edge
of SW1 will be synchronized to the rising edge of the
R
oscillators to maintain a 180 degree phase between SW1
and SW2, and also adjust slope compensation to avoid
subharmonic oscillation.
The synchronizing clock signal input to the LT3501 must
have a frequency between 250kHz and 1.5MHz, a duty
cycle between 20% and 80%, a low state below 0.5V and
a high state above 1.6V. Synchronization signals outside
of these parameters will cause erratic switching behavior.
The R
running frequency ((V
approximately equal to the synchronization frequency. If
the synchronization signal is halted, the synchronization
detection circuitry will timeout in typically 10µs at which
18
T
/SYNC pin signal. An AGC loop will adjust the internal
T
T
/SYNC pin can be used to synchronize the regulators
/SYNC resistor should be set such that the free
PL
) across the feedback divider may improve
C
pin (C
RT/SYNC
C
R
) integrates the error ampli-
C
C
C
– V
C
F
SYNCLO
LT3501
V
C
CURRENT MODE
POWER STAGE
g
T
m
3.6M
/SYNC resistor
)/R
= 3mho
Figure 6. Model for Loop Response
RT/SYNC
g
ERROR
m
AMP
= 275µmho
–
+
) is
C
+
.
0.8V
SW
FB
time the LT3501 reverts to the free-running frequency
based on the current through R
resistor is held above 1.6V at any time, switching will be
disabled.
If the synchronization signal is not present during regula-
tor start-up (for example, the synchronization circuitry is
powered from the regulator output) the R
see an equivalent resistance to ground between 15.4k and
133k until the synchronization circuitry is active for proper
start-up operation.
If the synchronization signal powers up in an undetermined
state (V
to the LT3501 as shown in Figure 7. The circuit as shown
will isolate the synchronization signal when the output
voltage is below 90% of the regulated output. The LT3501
will start-up with a switching frequency determined by the
resistor from the R
If the synchronization signal powers up in a low impedance
state (V
and the synchronizing clock. The equivalent resistance
seen from the R
frequency.
Figure 7. Synchronous Signal Powered from Regulator’s Output
R1
R2
OL
OL
, V
), connect a resistor between the R
TANTALUM
POLYMER
C
OH
PL
PG1
OR
, Hi-Z), connect the synchronization clock
T
LT3501
/SYNC pin to ground will set the start-up
R
T
T
ESR
/SYNC
V
3501 F06
/SYNC pin to ground.
OUT1
C1
OUTPUT
CERAMIC
C1
T
V
SYNCHRONIZATION
/SYNC. If the R
CC
CIRCUITRY
CLK
T
/SYNC pin must
3501 F07
T
/SYNC pin
T
/SYNC
3501fb