Datasheet

LTC1871-1
10
18711fb
When the oscillator’s internal logic circuitry detects a
synchronizing signal on the MODE/SYNC pin, the in-
ternal oscillator ramp is terminated early and the slope
compensation is increased by approximately 30%. As
a result, in applications requiring synchronization, it is
recommended that the nominal operating frequency of
the IC be programmed to be about 75% of the external
clock frequency. Attempting to synchronize to too high an
external frequency (above 1.3f
O
) can result in inadequate
slope compensation 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.
0.6V, and the current that fl ows into the FREQ pin is used
to charge and discharge an internal oscillator capacitor. A
graph for selecting the value of R
T
for a given operating
frequency is shown in Figure 6.
APPLICATIONS INFORMATION
Figure 5. MODE/SYNC Clock Input and Switching
Waveforms for Synchronized Operation
Figure 6. Timing Resistor (R
T
) Value
18711 F05
2V TO 7V
MODE/
SYNC
GATE
I
L
t
MIN
= 25ns
0.8T
D = 40%
T T = 1/f
O
Programming the Operating Frequency
The choice of operating frequency and inductor value is
a tradeoff between effi ciency and component size. Low
frequency operation improves effi ciency by reducing
MOSFET and diode switching losses. However, lower
frequency operation requires more inductance for a given
amount of load current.
The LTC1871-1 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
FREQUENCY (kHz)
100
R
T
(kΩ)
300
1000
18711 F06
10
100
200 1000
900
800700600
500
400
0
INTV
CC
Regulator Bypassing and Operation
An internal, P-channel low dropout voltage regulator pro-
duces the 5.2V supply which powers the gate driver and
logic circuitry within the LTC1871-1, as shown in Figure 7.
The INTV
CC
regulator can supply up to 50mA and must be
bypassed to ground immediately adjacent to the IC pins
with a minimum of 4.7µF 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 LTC1871-1 is redundant and the INTV
CC
pin can be shorted directly to the V
IN
pin. With the INTV
CC
pin shorted to V
IN
, however, the divider that programs the
regulated INTV
CC
voltage will draw 10µA of current from
the input supply, even in shutdown mode. For applications
that require the lowest shutdown mode input supply cur-
rent, do not connect the INTV
CC
pin to V
IN
. Regardless of
whether the INTV
CC
pin is shorted to V
IN
or not, it is always
necessary to have the driver circuitry bypassed with a
4.7μF tantalum or low ESR ceramic capacitor to ground
immediately adjacent to the INTV
CC
and GND pins.
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the power MOSFET.