LTC3704 Datasheet, PDF (10/28 Pages) Linear Technology – Wide Input Range, No RSENSE Positive-to-Negative DC/DC Controller

LTC3704
APPLICATIO S I FOR ATIO
MODE/
SYNC
t_MIN = 25ns
2V TO 7V
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
0.8T T T = 1/f_O
logic circuitry within the LTC3704, as shown in Figure 7.
GATE
D = 40%
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.
I_SW
3404 F05
Figure 5. MODE/SYNC Clock Input and Switching
Waveforms for Synchronized Operation
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 LTC3704 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
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.
1000
100
10
10
0 100 200 300 400 500 600 700 800 900 1000
FREQUENCY (kHz)
3704 F06
Figure 6. Timing Resistor (R_T) Value
–
1.230V
+
R2
LOGIC
INPUT
V_IN
SUPPLY
2.5V TO
30V
P-CH
R1
5.2V INTVCC
DRIVER
GATE
C_IN
+
C_VCC
4.7µF
M1
GND
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS 3704 F07
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
For input voltages that don’t exceed 7V (the absolute
maximum rating for this pin), the internal low dropout
regulator in the LTC3704 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
current, 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.
As a result, high input voltage applications in which a large
power MOSFET is being driven at high frequencies can
sn3704 3704fs