FAN21SV04 Fairchild Semiconductor, FAN21SV04 Datasheet - Page 12

• Manufacturer Part Number: FAN21SV04
• Description: This part is covered in Fairchild's href="http://www
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN21SV04 • Rev. 1.0.2
Circuit Operation
PWM Generation
Refer to Figure 2 for the PWM control mechanism.
FAN21SV04 uses the summing-mode method of control
to generate the PWM pulses. An amplified current-
sense signal is summed with an internally generated
ramp and the combined signal is compared with the
output of the error amplifier to generate the pulse width
to drive the high-side MOSFET. Sensed current from the
previous cycle is used to modulate the output of the
summing block. The output of the summing block is also
compared against a voltage threshold set by the R
resistor to limit the inductor current on a cycle-by-cycle
basis. R
the internal ramp and provides input voltage feed-
forward function. The controller facilitates external
compensation for enhanced flexibility.
Initialization
Once VIN_Reg voltage exceeds the UVLO threshold
and EN is HIGH, the IC checks for a shorted FB pin
before releasing the internal soft-start ramp (SS).
If the parallel combination of R1 and R
internal SS ramp is not released and the regulator does
not start.
Enable
FAN21SV04 has an internal pull-up to the enable (EN)
pin so that the IC is enabled once VIN_Reg exceeds the
UVLO threshold. Connecting a small capacitor across
EN and AGND delays the rate of voltage rise on the EN
pin. The EN pin also serves for the restart whenever a
fault occurs (refer to the Auto-Restart section). If the
regulator is enabled externally, the external EN signal
should go HIGH only after 5V_Reg is established. For
applications where such sequencing is required,
FAN21SV04 can be enabled (after the V
with external control, as shown in Figure 30.
If auto-restart is not desired, tie the EN pin HIGH with a
logic gate to keep the 1µA current sink from discharging
EN to 1.1V. Figure 32 shows one method to pull up EN
to V
CC
Figure 30.
for a latch configuration.
RAMP
resistor helps set the charging current for
Enabling with External Control
BIAS
CC
is  1k, the
comes up)
LIM
12
Internal Regulator
FAN21SV04 facilitates single-supply operation for input
voltages >6.5V. At startup, the output of the internal
regulator tracks the input voltage and comes into
regulation (5V) when VIN_Reg exceeds the UVLO
threshold. The EN pin is released at the same time. The
output voltage of the internal regulator (5V_Reg) is set
to 5V. The internal regulator supplies power to all the
control circuits including the drivers.
For applications with V
used if VIN_Reg is provided with a separate low-power
source >6.5V. VIN_Reg supply should come up after V
during dual-supply operation. The VIN_Reg pin should
always be decoupled with at least a 10 resistor and a
1µF ceramic capacitor (see Figure 10, Figure 11).
Since 5V_Reg is used to drive the internal MOSFET
gates, high peak currents are present on the 5V_Reg
pin. Connect a >2.2µf X5R or X7R decoupling capacitor
between the 5V_Reg pin and AGND. For V
operation, use a 3.3resistor in series with the boot
capacitor to reduce noise into the regulator.
In addition to supplying power for the control circuits
internally, 5V_Reg output can be used as a reference
voltage for other applications requiring low noise
reference voltage. 5V_Reg is capable of sourcing up to
5mA of output current.
When EN is pulled LOW externally, 5V_Reg output is
still present, but the IC is in standby mode with no
switching.
Soft-Start
FAN21SV04 uses an internal digital soft-start circuit to
slowly ramp up the output voltage and limit inrush
current during startup. When 5V_Reg is in regulation
and EN is HIGH, the circuit releases SS and enables the
PWM regulator. Soft-start time is a function of the
switching frequency (number of clock cycles).
Once internal SS ramp has charged to 0.8V (T0.8), the
output voltage is in regulation. Until SS ramp reaches
1.0V (T1.0), only the over-current-protection circuit is
active during soft-start and all other output protections
are inhibited.
In dual-supply operation mode, it is necessary to apply
VIN before VIN_Reg reaches its UVLO threshold to
avoid skipping the soft-start cycle.
VIN_Reg UVLO or toggling the EN pin discharges the
SS and resets the IC.
IN
<6.5V, FAN21SV04 can be
www.fairchildsemi.com
IN
>20V
IN