LTC1871 Linear Technology, LTC1871 Datasheet - Page 10

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

Programming the Operating Frequency
The choice of operating frequency and inductor value is a
tradeoff between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET and diode switching losses. However, lower
frequency operation requires more inductance for a given
amount of load current.
The LTC1871 uses a constant frequency architecture that
can be programmed over a 50kHz to 1000kHz range with
a single external resistor from the FREQ pin to ground, as
shown in Figure 1. The nominal voltage on the FREQ pin is
0.6V, and the current that flows into the FREQ pin is used
APPLICATIO S I FOR ATIO
LTC1871
When the oscillator’s internal logic circuitry detects a
synchronizing signal on the MODE/SYNC pin, the internal
oscillator ramp is terminated early and the slope compen-
sation is increased by approximately 30%. As a result, in
applications requiring synchronization, it is recommended
that the nominal operating frequency of the IC be pro-
grammed to be about 75% of the external clock frequency.
Attempting to synchronize to too high an external fre-
quency (above 1.3f
pensation and possible subharmonic oscillation (or jitter).
The external clock signal must exceed 2V for at least 25ns,
and should have a maximum duty cycle of 80%, as shown
in Figure 5. The MOSFET turn on will synchronize to the
rising edge of the external clock signal.
10
MODE/
SYNC
GATE
I
Figure 5. MODE/SYNC Clock Input and Switching
Waveforms for Synchronized Operation
L
t
MIN
O
U
= 25ns
) can result in inadequate slope com-
U
D = 40%
0.8T
W
T
T = 1/f
1871 F05
O
2V TO 7V
U
INTV
An internal, P-channel low dropout voltage regulator pro-
duces the 5.2V supply which powers the gate driver and
logic circuitry within the LTC1871, as shown in Figure 7.
The INTV
bypassed to ground immediately adjacent to the IC pins
with a minimum of 4.7 F tantalum or ceramic capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate driver.
For input voltages that don’t exceed 7V (the absolute
maximum rating for this pin), the internal low dropout
regulator in the LTC1871 is redundant and the INTV
can be shorted directly to the V
shorted to V
regulated INTV
the input supply, even in shutdown mode. For applications
that require the lowest shutdown mode input supply
current, do not connect the INTV
of whether the INTV
always necessary to have the driver circuitry bypassed
with a 4.7 F tantalum or low ESR ceramic capacitor to
ground immediately adjacent to the INTV
pins.
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the power MOSFET.
to charge and discharge an internal oscillator capacitor. A
graph for selecting the value of R
frequency is shown in Figure 6.
CC
Regulator Bypassing and Operation
CC
1000
100
10
regulator can supply up to 50mA and must be
Figure 6. Timing Resistor (R
IN
0
, however, the divider that programs the
100
CC
voltage will draw 10 A of current from
200
CC
300
FREQUENCY (kHz)
pin is shorted to V
400
500
600
IN
CC
pin. With the INTV
700
T
pin to V
for a given operating
800
T
) Value
900
1871 F06
1000
IN
IN
CC
. Regardless
or not, it is
and GND
CC
CC
pin
pin