LT1617 Linear Technology, LT1617 Datasheet - Page 4

• Manufacturer Part Number: LT1617
• Description: Constant-Current/ Constant-Voltage 1.4MHz Step-Up DC/DC Converter
• Manufacturer: Linear Technology

LT1617/LT1617-1
BLOCK DIAGRA
OPERATIO
The LT1617 uses a constant off-time control scheme to
provide high efficiencies over a wide range of output
current. Operation can be best understood by referring to
the block diagram in Figure 1. Q1 and Q2 along with R3 and
R4 form a bandgap reference used to regulate the output
voltage. When the voltage at the NFB pin is slightly below
–1.23V, comparator A1 disables most of the internal
circuitry. Output current is then provided by capacitor C2,
which slowly discharges until the voltage at the NFB pin
goes above the hysteresis point of A1 (typical hysteresis
at the NFB pin is 8mV). A1 then enables the internal
circuitry, turns on power switch Q3, and the current in
4
(EXTERNAL)
(EXTERNAL)
U
R1
R2
V
IN
V
OUT
NFB
W
C1
3
* 12mV FOR LT1617-1
5
Q1
R5
80k
V
IN
Q2
X10
R6
80k
R3
60k
R4
280k
+
–
Figure 1. LT1617 Block Diagram
4
SHDN
A1
ONE-SHOT
ENABLE
400ns
RESET
L1
inductors L1 and L2 begins ramping up. Once the switch
current reaches 350mA, comparator A2 resets the one-
shot, which turns off Q3 for 400ns. L2 continues to deliver
current to the output while Q3 is off. Q3 turns on again and
the inductor currents ramp back up until the switch
current reaches 350mA, then A2 again resets the one-
shot. This switching action continues until the output
voltage is charged up (until the NFB pin reaches –1.23V),
then A1 turns off the internal circuitry and the cycle
repeats. The LT1617-1 operates in the same manner,
except the switch current is limited to 100mA (the A2
reference voltage is 12mV instead of 42mV).
A2
+
–
DRIVER
42mV*
1
Q3
0.12
SW
C3
2
GND
D1
1617/-1 BD
L2
C2
V
OUT