LTC1628IG-PG Linear Technology, LTC1628IG-PG Datasheet - Page 10

• Manufacturer Part Number: LTC1628IG-PG
• Description: IC REG SW 2PHASE STEPDOWN 28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1628IG-PG

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: Adj to 0.8V
• Current - Output: 3A
• Frequency - Switching: 220kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATIO
LTC1628/LTC1628-PG
Main Control Loop
The LTC1628 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal opera-
tion, each top MOSFET is turned on when the clock for that
channel sets the RS latch, and turned off when the main
current comparator, I
inductor current at which I
trolled by the voltage on the I
each error amplifier EA. The V
voltage feedback signal, which is compared to the internal
reference voltage by the EA. When the load current in-
creases, it causes a slight decrease in V
the 0.8V reference, which in turn causes the I
increase until the average inductor current matches the
new load current. After the top MOSFET has turned off, the
bottom MOSFET is turned on until either the inductor
current starts to reverse, as indicated by current compara-
tor I
The top MOSFET drivers are biased from floating boot-
strap capacitor C
each off cycle through an external diode when the top
MOSFET turns off. As V
V
the top MOSFET continuously. The dropout detector de-
tects this and forces the top MOSFET off for about 500ns
every tenth cycle to allow C
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 1.2 A
current source to charge soft-start capacitor C
C
I
value. As C
gradually released allowing normal, full-current opera-
tion. When both RUN/SS1 and RUN/SS2 are low, all
LTC1628 controller functions are shut down, and the
STBYMD pin determines if the standby 5V and 3.3V
regulators are kept alive.
Low Current Operation
The FCB pin is a multifunction pin providing two func-
tions: 1) to provide regulation for a secondary winding by
temporarily forcing continuous PWM operation on
10
TH
OUT
SS
voltage clamped at approximately 30% of its maximum
reaches 1.5V, the main control loop is enabled with the
2
, the loop may enter dropout and attempt to turn on
, or the beginning of the next cycle.
SS
continues to charge, the I
B
U
, which normally is recharged during
(Refer to Functional Diagram)
1
, resets the RS latch. The peak
IN
decreases to a voltage close to
1
B
TH
resets the RS latch is con-
to recharge.
pin, which is the output of
OSENSE
OSENSE
pin receives the
TH
pin voltage is
TH
relative to
voltage to
SS
. When
controller 1 (or both controllers depending upon the
FLTCPL pin); and 2) select between two modes of low
current operation. When the FCB pin voltage is below
0.800V, the controller forces continuous PWM current
mode operation. In this mode, the top and bottom
MOSFETs are alternately turned on to maintain the output
voltage independent of direction of inductor current.
When the FCB pin is below V
0.80V, the controller enters Burst Mode operation. Burst
Mode operation sets a minimum output current level
before inhibiting the top switch and turns off the synchro-
nous MOSFET(s) when the inductor current goes nega-
tive. This combination of requirements will, at low cur-
rents, force the I
temporarily inhibit turn-on of both output MOSFETs until
the output voltage drops. There is 60mV of hysteresis in
the burst comparator B tied to the I
produces output signals to the MOSFETs that turn them
on for several cycles, followed by a variable “sleep”
interval depending upon the load current. The resultant
output voltage ripple is held to a very small value by
having the hysteretic comparator after the error amplifier
gain block.
Constant Frequency Operation
When the FCB pin is tied to INTV
is disabled and the forced minimum output current re-
quirement is removed. This provides constant frequency,
discontinuous (preventing reverse inductor current) cur-
rent operation over the widest possible output current
range. This constant frequency operation is not as efficient
as Burst Mode operation, but does provide a lower noise,
constant frequency operating mode down to approxi-
mately 1% of designed maximum output current.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking current
while in forced continuous operation, current will be
forced back into the main power supply potentially boost-
ing the input supply to dangerous voltage levels—
BEWARE!
TH
pin below a voltage threshold that will
INTVCC
CC
, Burst Mode operation
TH
– 2V but greater than
pin. This hysteresis
1628fb