FAN48632 Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – 2.5 MHz, 2.0 A Pulsed-Load Synchronous TinyBoost

Circuit Description
FAN48632 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 set point.
In anticipation of a heavy load transition, the set point 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 2. Operating States
Mode
Description
Invoked When
LIN
SS
BST
BPS
Linear Startup
Boost Soft-Start
Boost Operating Mode
True Bypass Mode
VIN > VOUT
VOUT < VOUT(MIN)
VOUT = VOUT(MIN)
VIN > VOUT(MIN)
Boost Mode
The FAN48632 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 3. Boost Startup Sequence
Start
State
Entry
Exit
End Timeout
State (µs)
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 s 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 s, 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 µs, 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.
Once a fault is triggered, the regulator stops switching and
presents a high-impedance path between VIN and VOUT. After
waiting 20 ms, a restart is attempted.
Power Good
Power good is 0 FAULT, 1 POWER GOOD, open-drain input.
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 µs OR the die the temperature exceeds 120°C. PG
is re-asserted when the device cools below to 100°C.
 Any FAULT condition causes PG to be de-asserted.
Over-Temperature
The regulator shuts down when the die temperature exceeds
150°C. Restart occurs when the IC has cooled by
approximately 20°C.
© 2013 Fairchild Semiconductor Corporation
FAN48632 • Rev. 1.0.3
9
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