Datasheet
LT3970 Series
8
3970fc
pin FuncTions
FB (Pin 1, LT3970 Only): The LT3970 Regulates the FB
Pin to 1.21V. Connect the feedback resistor divider tap to
this pin.
V
OUT
(Pin 1, LT3970-X Only): The LT3970-3.3, LT3970-3.42
and LT3970-5 regulate the V
OUT
Pin to 3.3V, 3.42V and 5V
respectively. This pin connects to the internal feedback
divider that programs the fixed output voltage.
EN (Pin 2): The part is in shutdown when this pin is low
and active when this pin is high. The hysteretic threshold
voltage is 1V going up and 0.97V going down. Tie to V
IN
if shutdown feature is not used. The EN threshold is ac-
curate only when V
IN
is above 4.2V. If V
IN
is lower than
4.2V, ground EN to place the part in shutdown.
V
IN
(Pin 3): The V
IN
pin supplies current to the LT3970’s
internal circuitry and to the internal power switch. This
pin must be locally bypassed.
GND (Pins 4, 5, Exposed Pad (Pin 11, DFN Only)): Ground.
Must be soldered to PCB.
SW (Pin 6): The SW pin is the output of an internal power
switch. Connect this pin to the inductor.
BOOST (Pin 7): This pin is used to provide a drive volt
-
age, higher than the input voltage, to the internal bipolar
NPN power switch.
BD (Pin 8): This pin
connects to the anode of the boost
diode. This pin also supplies current to the LT3970’s internal
regulator when BD is above 3.2V.
PG (Pin 9): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within 10% of the final regulation voltage. PG is valid when
V
IN
is above 4.2V and EN is high.
RT (Pin 10): A resistor is tied between RT and ground to
set the switching frequency.
block Diagram
R
SWITCH LATCH
D
BOOST
D
CATCH
BOOST
OSCILLATOR
200kHz TO 2.2MHz
SLOPE COMP
S
Q
–
+
–
+
–
+
Burst Mode
DETECT
ERROR
AMP
1.09V
SHDN
EN
1V
C1
V
IN
INTERNAL 1.21V REF
–
+
2
RT
R
T
10
PG
9
FB
LT3970 ONLY
* LT3970-3.3: R1 = 12.65M, R2 = 7.35M
LT3970-3.42: R1 = 12.65M, R2 = 6.93M
LT3970-5: R1 = 15.15M, R2 = 4.85M
R2 R1
R2 R1
V
C
1
GND
(4, 5)
V
IN
3
7
BD
8
SW
V
OUT
6
C2
C3
3990 BD
L1
V
OUT
LT3970-X
ONLY*
1