9
Rev. C
BLOCK DIAGRAM
OPERATION
The LT8607 is a monolithic constant frequency current
mode step-down DC/DC converter. An oscillator with
frequency set using a resistor on the RT pin turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which the
top switch turns off is controlled by the voltage on the
internal VC node. The error amplifier servos the VC node
by comparing the voltage on the V
FB
pin with an inter-
nal 0.778V reference. When the load current increases it
causes a reduction in the feedback voltage relative to the
reference leading the error amplifier to raise the VC volt-
age until the average inductor current matches the new
load current. When the top power switch turns off the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in excess current flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
If the EN/UV pin is low, the LT8607 is shut down and
draws 1µA from the input. When the EN/UV pin is above
1.05V, the switching regulator becomes active.
To optimize efficiency at light loads, the LT8607 enters
Burst Mode operation during light load situations. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
1.7µA. In a typical application, 3.0µA will be consumed
from the input supply when regulating with no load. The
SYNC pin is tied low to use Burst Mode operation and can
be floated to use pulse-skipping mode. If a clock is applied
to the SYNC pin the part will synchronize to an external
clock frequency and operate in pulse-skipping mode. While
in pulse-skipping mode the oscillator operates continu-
ously and positive SW transitions are aligned to the clock.
During light loads, switch pulses are skipped to regulate
the output and the quiescent current will be several mA.
The SYNC pin may be tied high for spread spectrum modu-
lation mode, and the LT8607 will operate similar to pulse-
skipping mode but vary the clock frequency to reduce EMI.
The LT8607 DFN has no SYNC pin and will always operate
in Burst Mode operation. The LT8607B has no SYNC pin
and will operate in pulse-skipping mode.
Comparators monitoring the FB pin voltage will pull the PG
pin low if the output voltage varies more than ±8.5% (typi-
cal) from the set point, or if a fault condition is present.
The oscillator reduces the LT8607’s operating frequency
when the voltage at the FB pin is low and the part is in Burst
Mode operation. This frequency foldback helps to control
the inductor current when the output voltage is lower than
the programmed value which occurs during start-up.
+
+
–
+
–
SLOPE COMP
INTERNAL 0.778V REF
OSCILLATOR
200kHz TO 2.2MHz
BURST
DETECT
3.5V
REG
M1
M2
C
BST
C
OUT
V
OUT
8607 BD
SW
L
BST
SWITCH
LOGIC
AND
ANTI-
SHOOT
THROUGH
ERROR
AMP
SHDN
±8.5%
V
C
SHDN
TSD
INTV
CC
UVLO
V
IN
UVLO
SHDN
TSD
V
IN
UVLO
EN/UV
1V
+
–
INTV
CC
GND
PG
FB
R1
R
PG
R2
R
T
C
SS
TR/SS (MSOP ONLY)
2µA
RT
SYNC (MSOP ONLY)
V
IN
V
IN
V
OUT
C
IN
C
FF
C
VCC
R3
OPT
R4
OPT
