4
LT3750
3750fa
UU
U
PI FU CTIO S
V
TRANS
(Pin 1): Transformer Supply Pin. Powers the
primary coil of the transformer as well as internal circuitry
that performs boundary mode detection. Bypass at the pin
with a 1µF to 10µF capacitor. Bypass the primary winding
of the transformer with a large capacitor.
DONE (Pin 2): Open Collector Indication Pin. When target
output voltage is reached, an NPN transistor turns on.
Requires a pull-up resistor or current source. Any fault
conditions such as thermal shutdown or undervoltage
lockout will also turn on the NPN.
CHARGE (Pin 3): Charge Pin. Initiates a new charge cycle
when brought high or discontinues charging and puts part
into shutdown when low. To properly enable the device, a
step input with a minimum ramp rate of 1V/µs is required.
Drive to 1.1V or higher to enable the device; drive below
0.2V to disable the device.
V
CC
(Pin 4): Input Supply Pin. Bypass locally with a
ceramic capacitor. A 1µF to 10µF ceramic capacitor should
be sufficient for most applications.
GND (Pin 5): Ground Pin. Connect directly to local ground
plane.
SOURCE (Pin 6): Source Pin. Senses NMOS drain current.
Connect NMOS source terminal and current sense resistor
to this pin. The current limit is 78mV/R
SENSE
.
GATE (Pin 7): Gate Pin. Connect NMOS gate terminal to
this pin. Internal gate driver will drive voltage to within
V
CC
– 2V during each switching cycle.
RDCM (Pin 8): Discontinuous Mode Sense Pin. Senses
when current in transformer has decayed to zero and ini-
tiates a new charge cycle if output voltage target has not
been reached. Place a resistor between this pin and the drain
of the NMOS. A good choice is a 43k, 5% resistor.
RV
OUT
(Pin 9): Output Voltage VI Converter Pin. Develops
a current proportional to output capacitor voltage. Con-
nect a resistor between this pin and the drain of the NMOS.
RBG (Pin 10): Output Voltage Sense Pin. Senses the
voltage across the RBG resistor, which is proportional to
the current flowing into the R
VOUT
pin. When voltage
equals 1.24V, charging is disabled and DONE pin goes
low. Connect a resistor (2.5k or less is recommended)
from this pin to GND. A 2.49k, 1% resistor is a good
choice.
