LTC1871IMS-1#TRPBF Linear Technology, LTC1871IMS-1#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LTC1871IMS-1#TRPBF
• Description: IC CONTRLR CURRENT MODE 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LTC1871IMS-1#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 72 V
• Current - Output: 50mA
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 2.5 ~ 36 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LTC1871-1
APPLICATIONS INFORMATION
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
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
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.
10
MODE/
SYNC
GATE
I
Figure 5. MODE/SYNC Clock Input and Switching
Waveforms for Synchronized Operation
L
t
MIN
= 25ns
D = 40%
O
0.8T
) can result in inadequate
T
T = 1/f
18711 F05
O
2V TO 7V
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
frequency is shown in Figure 6.
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 LTC1871-1, as shown in Figure 7.
The INTV
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
pin can be shorted directly to the V
pin shorted to V
regulated INTV
the input supply, even in shutdown mode. For applications
that require the lowest shutdown mode input supply cur-
rent, do not connect the INTV
whether the INTV
necessary to have the driver circuitry bypassed with a
4.7μF tantalum or low ESR ceramic capacitor to ground
immediately adjacent to the INTV
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the power MOSFET.
CC
Regulator Bypassing and Operation
CC
1000
100
10
regulator can supply up to 50mA and must be
Figure 6. Timing Resistor (R
0
CC
100
IN
CC
, however, the divider that programs the
voltage will draw 10μA of current from
200
pin is shorted to V
300
FREQUENCY (kHz)
400
500
CC
600
pin to V
700
T
IN
CC
for a given operating
800
pin. With the INTV
IN
T
and GND pins.
) Value
or not, it is always
900
18711 F06
IN
1000
. Regardless of
18711fb
CC
CC