ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 11

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATION
Table 1 summarizes the differences between the LTC3858-2
and its sister parts, the LTC3858 and LTC3858-1. The
LTC3858-2 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
which is the output of the error amplifier, EA. The error
amplifier compares the output voltage feedback signal at
the V
divider connected across the output voltage, V
ground) to the internal 0.800V reference voltage. When the
load current increases, it causes a slight decrease in V
relative to the reference, which causes the EA to increase
the I
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.
Table 1. Summary of the Differences Between LTC3858, LTC3858-1 and LTC3858-2
Short-Circuit Latchoff Feature?
Overvoltage Protection
Optional Tracking Start-Up?
Independent PGOOD1 and PGOOD2 Pins?
CLKOUT/PHASMD Pins for PolyPhase
Adjustable Current Limit (I
Package
TH
FB
voltage until the average inductor current matches
pin (which is generated with an external resistor
LIM
(Refer to the Functional Diagram)
Pin)
(Bottom MOSFET “Crowbar”)
Yes, But Can Be Defeated
5mm × 5mm QFN
BG Forced On
LTC3858
Yes
Yes
Yes
No
OUT
TH
pin,
, to
FB
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
the EXTV
4.7V, the V
5.1V from V
the V
on. Once enabled, the EXTV
EXTV
power to be derived from a high efficiency external source
such as one of the LTC3858-2 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, C
switching cycle through an external diode when the top
MOSFET turns off. If the input voltage, V
a voltage close to V
attempt to turn on the top MOSFET continuously. The
dropout detector detects this and forces the top MOSFET
off for about one-twelfth of the clock period every tenth
cycle to allow C
(Bottom MOSFET “Crowbar”)
Yes, But Can Be Defeated
4mm × 5mm QFN and
CC
No, PGOOD1 Only
IN
CC
BG Forced On
Narrow SSOP
/EXTV
LTC3858-1
LDO is turned off and the EXTV
to INTV
CC
IN
No
No
No
pin is left open or tied to a voltage less than
IN
LDO (low dropout linear regulator) supplies
CC
to INTV
CC
B
Power
B
. Using the EXTV
to recharge.
, which normally recharges during each
OUT
CC
, the loop may enter dropout and
. If EXTV
(Behavior Depends on Light Load Mode)
CC
LDO supplies 5.1V from
CC
CC
BG Not Forced On
5mm × 5mm QFN
LTC3858-2
pin allows the INTV
Not Present
LTC3858-2
is taken above 4.7V,
Yes
Yes
Yes
Yes
CC
IN
LDO is turned
, decreases to
CC
pin. When
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CC