FAN48630J
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10
CIRCUIT DESCRIPTION
FAN48630J is a synchronous boost regulator, typically
operating at 2.5 MHz in Continuous Conduction Mode
(CCM), which occurs at moderate to heavy load current and
low VIN voltages. The regulator includes a Bypass Mode
that activates when VIN is above the boost regulator’s
setpoint.
In anticipation of a heavy load transition, the setpoint can
be adjusted upward by fixed amounts with the VSEL pin to
reduce the required system headroom during lighter−load
operation to save power.
Table 9. Operating States
Mode Description Invoked When
LIN Linear Startup VIN > VOUT
SS Boost Soft−Start VOUT < VOUT(MIN)
BST Boost Operating Mode VOUT = VOUT(MIN)
BPS True Bypass Mode VIN > VOUT(MIN)
Boost Mode
The FAN48630J uses a current−mode modulator to
achieve excellent transient response and smooth transitions
between CCM and Discontinuous Conduction Mode
(DCM) operation. During CCM operation, the device
maintains a switching frequency of about 2.5 MHz.
In light−load operation (DCM), frequency is reduced to
maintain high efficiency.
Table 10. Boost Startup Sequence
Start
State Entry Exit End State
Timeout
(s)
LIN1 VIN >
UVLO,
EN = 1
VOUT >
VIN−300 mV
SS
LIN2 512
LIN2 LIN1 Exit VOUT >
VIN−300 mV
SS
TIMEOUT FAULT 1024
SS LIN1 or
LIN2 Exit
VOUT =
VOUT(MIN)
BST
OVERLOAD
TIMEOUT
FAULT 64
Shutdown and Startup
If EN is LOW, all bias circuits are off and the regulator is
in Shutdown Mode. During shutdown, current flow is
prevented from VIN to VOUT, as well as reverse flow from
VOUT to VIN. During startup, it is recommended to keep DC
current draw below 500 mA.
LIN State
When EN is HIGH and VIN > UVLO, the regulator
attempts to bring VOUT within 300 mV of VIN using the
internal fixed current source from VIN (Q3). The current is
limited to LIN1 set point.
If VOUT reaches VIN−300 mV during LIN1 Mode, the SS
state is initiated. Otherwise, LIN1 times out after 512 ms and
LIN2 Mode is entered.
In LIN2 Mode, the current source is incremented to 2 A.
If VOUT fails to reach VIN−300 mV after 1024 ms, a fault
condition is declared.
SS State
Upon the successful completion of the LIN state
(VOUT ≥VIN−300 mV), the regulator begins switching with
boost pulses current limited to 50% of nominal level.
During SS state, VOUT is ramped up by stepping the
internal reference. If VOUT fails to reach regulation during
the SS ramp sequence for more than 64 ms, a fault condition
is declared. If large COUT is used, the reference
is automatically stepped slower to avoid excessive input
current draw.
BST State
This is a normal operating state of the regulator.
BPS State
If VIN is above VREG when the SS Mode successfully
completes, the device transitions directly to BPS Mode.
FAULT State
The regulator enters the FAULT state under any of
the following conditions:
•VOUT fails to achieve the voltage required to advance
from LIN state to SS state.
•VOUT fails to achieve the voltage required to advance
from SS state to BST state.
•Boost current limit triggers for 2 ms during the BST
state.
•VDS protection threshold is exceeded during BPS state.
•VIN drops below UVLO threshold.
Once a FAULT is triggered, the regulator stops switching
and presents a high−impedance path between VIN to VOUT.
After waiting 20 ms, a restart is attempted.
Power Good
Power good is 0 FAULT, 1 POWER GOOD, open−drain
output.
The Power good pin is provided for signaling the system
when the regulator has successfully completed soft−start
and no faults have occurred. Power good also functions as
an early warning flag for high die temperature and overload
conditions.
•PG is released HIGH when the soft−start sequence is
successfully completed.
•PG is pulled LOW when PMOS current limit has
triggered for 64 ms OR the die the temperature exceeds