FAN4860 Datasheet, PDF (10/15 Pages) Fairchild Semiconductor – 3MHz, 5V Output Synchronous TinyBoost™ Regulator

To ensure that VOUT does not pump significantly above the
regulation point, the boost switch remains off as long as
FB > VREF.
Fault State
The regulator enters the FAULT state under any of the
following conditions:
ƒ VOUT fails to achieve the voltage required to advance from
LIN state to SS state.
ƒ VOUT fails to achieve the voltage required to advance from
SS state to BST state.
ƒ Sustained (32 CLK counts) pulse-by-pulse current limit
during the BST state.
ƒ The regulator moves from BST to LIN state due to a short
circuit or output overload (VOUT < VIN-1V).
Once a fault is triggered, the regulator stops switching and
presents a high-impedance path between VIN and VOUT. After
waiting 480 CLK counts, a re-start is attempted.
Soft-Start and Fault Timing
The soft-start timing for each state, and the fault times, are
determined by the fault clock, whose period is inversely
proportional to VIN. This allows the regulator more time to
charge larger values of COUT when VIN is lower. With higher
VIN, this also reduces power delivered to VOUT during each
cycle in current limit.
The number of clock counts for each state is illustrated in
Figure 27.
The fault clock period as a function of VIN is shown in Figure 28.
Figure 28. Fault Clock Period vs. VIN
The VIN-dependent LIN mode charging current is illustrated
in Figure 29.
Figure 27. Fault Response into Short Circuit
Figure 29. LIN Mode Current vs. VIN
Over-Temperature Protection (OTP)
The regulator shuts down when the thermal shutdown
threshold is reached. Restart, with soft-start, occurs when
the IC has cooled by about 30°C.
Over-Current Protection (OCP)
During boost-mode operation, the FAN4860 employs a
cycle-by-cycle peak current limit to protect switching
elements. Sustained current limit, for 32 consecutive fault
CLK counts, initiates a fault condition.
During an overload condition, as VOUT collapses to
approximately VIN-1V, the synchronous rectifier is
immediately switched off and a fault condition is declared.
Automatic restart occurs once the overload/short is removed
and the fault timer completes counting.
© 2009 Fairchild Semiconductor Corporation
FAN4860 • Rev. 1.0.3
10
www.fairchildsemi.com