6
LT3420/LT3420-1
3420fb
UU
U
PI FU CTIO S
R
REF
(Pin 1): Reference Resistor Pin. Place a resistor (R2)
from the R
REF
pin to GND. 2k is recommended.
V
BAT
(Pin 2): Battery Voltage Input. This pin should be
connected to the power supply or battery, which supplies
power to transformer T1. Must be locally bypassed.
R
FB
(Pin 3): Feedback Resistor Pin. Place a resistor (R1)
from the SW pin to the R
FB
pin. Set R1 according to the
following formula:
R
R
N
RNV V
SEC OUT D
1
2
14 2
2
=++
[]
(. • ) ( )
(LT3420)
R
R
N
RNV V
SEC OUT D
1
2
2
2
=++
[]
()( )
(LT3420-1)
V
OUT
: Desired Output Voltage
N: Transformer Turns Ratio
R
SEC
: Transformer Secondary Resistance
V
D
: Diode Forward Voltage Drop
R2: Resistor from the R
REF
Pin to GND. 2k is a Typical
Choice
V
CC
(Pin 4): Input Supply Pin. Must be locally bypassed
with a 4.7µF or larger ceramic capacitor.
GND (Pin 5): Ground. Tie directly to local ground plane.
SW (Pin 6): Switch Pin. This is the collector of the internal
NPN power switch. Minimize the metal trace area con-
nected to this pin to minimize EMI.
SEC (Pin 7): Transformer Secondary Pin. Tie one end of
the transformer secondary to this pin. Take care to use the
correct phasing of the transformer (Refer to Figures 1
and 2).
DONE (Pin 8): Done Output Pin. Open collector NPN
output. DONE is pulled low whenever the chip is delivering
power to the output and goes high when power delivery
stops.
CHARGE (Pin 9): Charge Pin. Drive CHARGE high (1.5V or
more) to commence charging of the output capacitor.
Drive to 0.2V or less to put the part in shutdown mode.
C
T
(Pin 10): Refresh Timer Capacitor Pin. Place a capacitor
from the C
T
pin to GND to set the refresh timer sample rate
according to the following formula:
C
T
= 2.5 • 10
–6
• t
REFRESH
t
REFRESH
: Desired Refresh Period in Seconds.
EXPOSED PAD (Pin 11) (DD Package only): GND. Must be
soldered to local ground plane on PCB.