LTC3858-1 Datasheet, PDF (10/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3858-1
FUNCTIONAL Diagram
PGOOD1
FREQ
20µA
0.88V
V_FB1
0.72V
VCO
CLK2
CLK1
PFD
PLLIN/MODE
100k
SYNC
DET
V_IN
EXTV_CC
+
4.7V ^–
5.1V
LDO
EN
5.1V
LDO
EN
SGND
INTV_CC
DUPLICATE FOR SECOND
CONTROLLER CHANNEL
BOOST
INTV_CC V_IN
D_B
DROP
OUT
DET
BOT
SQ
TOP ON
TG
TOP
SW
RQ
SHDN
INTV_CC
SWITCH
LOGIC _BOT
BG
PGND
C_B
D
C_IN
C_OUT
0.425V ⁺
–
SLEEP
ICMP
–+
2(V_FB)
0.45V
+ ^IR
–
3mV
SENSE⁺
SENSE^–
SLOPE COMP
V_FB
EA 0.80V
TRACK/SS
OV
0.5µA
10V
SHDN
RST
2(V_FB)
FOLDBACK
0.88V
I_TH
1µA _SS
RUN
SHORT CKT
LATCH-OFF
SHDN
10µA
L
R_SENSE
R_B
R_A
C_C
C_C2 R_C
C_SS
38581 FD
V_OUT
Operation
Main Control Loop
The LTC3858-1 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal op-
eration, each external top MOSFET is turned on when the
clock for that channel sets the RS latch, and is turned off
when the main current comparator, ICMP, resets the RS
latch. The peak inductor current at which ICMP trips and
resets the latch is controlled by the voltage on the I_TH pin,
which is the output of the error amplifier, EA. The error
amplifier compares the output voltage feedback signal at
10
the V_FB pin (which is generated with an external resistor
divider connected across the output voltage, V_OUT , to
ground) to the internal 0.800V reference voltage. When the
load current increases, it causes a slight decrease in V_FB
relative to the reference, which causes the EA to increase
the I_TH voltage until the average inductor current matches
the new load current.
After the top MOSFET is turned off each cycle, the bottom
MOSFET is turned on until either the inductor current starts
to reverse, as indicated by the current comparator IR, or
the beginning of the next clock cycle.
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