ltc1871ems-trpbf Linear Technology Corporation, ltc1871ems-trpbf Datasheet - Page 11

• Manufacturer Part Number: ltc1871ems-trpbf
• Description: Wide Input Range, No Rsense Current Mode Boost, Flyback And Sepic Controller
• Manufacturer: Linear Technology Corporation

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 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.
1000
100
10
Figure 6. Timing Resistor (R
0
100
200
300
FREQUENCY (kHz)
400
1.230V
R2
500
600
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
700
T
–
+
LOGIC
for a given operating
800
T
) Value
900
1871 F06
1000
R1
DRIVER
P-CH
5.2V
INTV
GATE
GND
V
CC
IN
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 4.7μF 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 is redundant and the INTV
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.
As a result, high input voltage applications in which a
large power MOSFET is being driven at high frequencies
can cause the LTC1871 to exceed its maximum junction
1871 F07
+
CC
Regulator Bypassing and Operation
C
4.7μF
CC
VCC
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
regulator can supply up to 50mA and must be
CC
C
IN
IN
CC
, however, the divider that programs the
voltage will draw 10μA of current from
M1
pin is shorted to V
INPUT
SUPPLY
2.5V TO 30V
GND
CC
pin to V
IN
CC
pin. With the INTV
IN
and GND pins.
or not, it is always
LTC1871
IN
. Regardless of
11
CC
1871fe
pin
CC