LTC1871IMS-7#TRPBF Linear Technology, LTC1871IMS-7#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LTC1871IMS-7#TRPBF
• Description: IC CONTRLR CURRENT MODE 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LTC1871IMS-7#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 36 V
• Current - Output: 50mA
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 6 ~ 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: -

LTC1871-7
OPERATION
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.
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 LTC1871-7 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 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
produces the 7V supply which powers the gate driver and
10
MODE/
SYNC
CC
GATE
Figure 5. MODE/SYNC Clock Input and Switching
Waveforms for Synchronized Operation
I
L
Regulator Bypassing and Operation
t
MIN
= 25ns
D = 40%
O
0.8T
) can result in inadequate
T
for a given operating
T
T = 1/f
18717 F05
O
2V TO 7V
logic circuitry within the LTC1871-7, 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.
The LTC1871-7 contains an undervoltage lockout circuit
which protects the external MOSFET from switching at low
gate-to-source voltages. This undervoltage circuit senses
the INTV
4.6V falling threshold.
For input voltages that don’t exceed 8V (the absolute
maximum rating for INTV
regulator in the LTC1871-7 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
of whether the INTV
always necessary to have the driver circuitry bypassed
with a 4.7μF 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.
As a result, high input voltage applications in which a
large power MOSFET is being driven at high frequencies
CC
CC
1000
100
10
voltage and has a 5.6V rising threshold and a
regulator can supply up to 50mA and must be
Figure 6. Timing Resistor (R
0
CC
100
IN
, however, the divider that programs the
voltage will draw 14μA of current from
200
CC
CC
300
FREQUENCY (kHz)
pin is shorted to V
and GND pins.
CC
400
is 9V), the internal low dropout
500
600
CC
700
pin to V
IN
800
pin. With the INTV
T
) Value
900
18717 F06
1000
IN
IN
. Regardless
or not, it is
18717fc
CC
CC