ltc3787gn Linear Technology Corporation, ltc3787gn Datasheet - Page 14

• Manufacturer Part Number: ltc3787gn
• Description: Ltc3787 - Polyphase Synchronous Boost Controller
• Manufacturer: Linear Technology Corporation

LTC3787
operaTion
multiple LTC3787s can be configured for 2-, 3-, 4- , 6- and
12-phase operation.
Table 1.
CLKOUT is disabled when the controller is in shutdown
or in sleep mode.
Operation When V
When V
controller can behave differently depending on the mode,
inductor current and V
mode, the loop works to keep the top MOSFET on con-
tinuously once V
pump delivers current to the boost capacitor to maintain
a sufficiently high TG voltage. The amount of current the
charge pump can deliver is characterized by two curves
in the Typical Performance Characteristics section.
In pulse-skipping mode, if V
110% of the regulated V
inductor current rises above a certain threshold and turns
off if the inductor current falls below this threshold. This
threshold current is set to approximately 6%, 4% or
3% of the maximum ILIM current when the ILIM pin is
grounded, floating or tied to INTV
controller is programmed to Burst Mode operation under
this same V
the inductor current.
If V
any mode, the controller turns on TG regardless of the
inductor current. In Burst Mode operation, however, the
internal charge pump turns off if the chip is asleep. With
the charge pump off, there would be nothing to prevent
the boost capacitor from discharging, resulting in an
insufficient TG voltage needed to keep the top MOSFET
completely on. To prevent excessive power dissipation
across the body diode of the top MOSFET in this situation,
the chip can be switched over to forced continuous mode
to enable the charge pump or a Schottky diode can also
be placed in parallel to the top MOSFET.
14
V
Floating
INTV
IN
PHASMD
GND
rises above 110% of the regulated V
CC
IN
rises above the regulated V
IN
CONTROLLER 2 PHASE (°C)
window, then TG remains off regardless of
IN
IN
rises above V
> Regulated V
180
180
240
IN
OUT
voltage. In forced continuous
voltage, TG turns on if the
IN
OUT
is between 100% and
CC
OUT
OUT
. The internal charge
, respectively. If the
CLKOUT PHASE (°C)
voltage, the boost
OUT
120
60
90
voltage in
Power Good
The PGOOD pin is connected to an open drain of an
internal N-channel MOSFET. The MOSFET turns on and
pulls the PGOOD pin low when the VFB pin voltage is not
within ±10% of the 1.2V reference voltage. The PGOOD
pin is also pulled low when the corresponding RUN pin
is low (shut down). When the VFB pin voltage is within
the ±10% requirement, the MOSFET is turned off and the
pin is allowed to be pulled up by an external resistor to a
source of up to 6V (abs max).
Operation at Low SENSE Pin Common Mode Voltage
The current comparator in the LTC3787 is powered directly
from the SENSE
voltage of the SENSE
low as 2.5V, which is below the UVLO threshold. The figure
on the first page shows a typical application in which the
controller’s VBIAS is powered from V
supply can go as low as 2.5V. If the voltage on SENSE
drops below 2.5V, the SS pin will be held low. When the
SENSE voltage returns to the normal operating range, the
SS pin will be released, initiating a new soft-start cycle.
BOOST Supply Refresh and Internal Charge Pump
Each top MOSFET driver is biased from the floating
bootstrap capacitor, C
each cycle through an external diode when the bottom
MOSFET turns on. There are two considerations for keep-
ing the BOOST supply at the required bias level. During
start-up, if the bottom MOSFET is not turned on within
100μs after UVLO goes low, the bottom MOSFET will be
forced to turn on for ~400ns. This forced refresh gener-
ates enough BOOST-SW voltage to allow the top MOSFET
ready to be fully enhanced instead of waiting for the initial
few cycles to charge up. There is also an internal charge
pump that keeps the required bias on BOOST. The charge
pump always operates in both forced continuous mode
and pulse-skipping mode. In Burst Mode operation, the
charge pump is turned off during sleep and enabled when
the chip wakes up. The internal charge pump can normally
supply a charging current of 55μA.
+
pin. This enables the common mode
+
B
, which normally recharges during
and SENSE
–
pins to operate at as
OUT
while the V
3787fa
IN
+