FAN2315MPX Datasheet, PDF (11/19 Pages) Fairchild Semiconductor – FAN2315 TinyBuck 15 A Integrated Synchronous Buck Regulator

Circuit Operation
The FAN2315 uses a constant on-time modulation
architecture with a V_IN feed-forward input to
accommodate a wide V_IN range. This method provides
fixed switching frequency (f_SW) operation when the
inductor operates in Continuous Conduction Mode
(CCM) and variable frequency when operating in Pulse
Frequency Mode (PFM) at light loads. Additional
benefits include excellent line and load transient
response, cycle-by-cycle current limiting, and no loop
compensation required.
At the beginning of each cycle, FAN2315 turns on the
high-side MOSFET (HS) for a fixed duration (t_ON). At the
end of t_ON, HS turns off for a duration (t_OFF) determined
by the operating conditions. Once the FB voltage (V_FB)
falls below the reference voltage (V_REF), a new switching
cycle begins.
The modulator provides a minimum off-time (t_OFF-MIN) of
320 ns to provide a guaranteed interval for low-side
MOSFET (LS) current sensing and PFM operation. t_OFF-
MIN ^{is also used to provide stability against multiple}
pulsing and limits maximum switching frequency during
transient events.
Enable
The enable pin is TTL compatible, which supports low-
shutdown-current applications, such as notebooks. If an
external enable signal is not available or not required,
the FAN2315 can be self-enabled by connecting the EN
pin to V_CC. V_CC should be applied after V_IN / PV_IN is
applied to the circuit.
The EN pin can be directly driven by logic voltages of
5 V, 3.3 V, 2.5 V, etc. If the EN pin is driven by 5V logic,
a small current flows into the pin when the EN pin
voltage exceeds the internal clamp voltage of 4.3 V. To
eliminate clamp current flowing into the EN pin use a
voltage divider to limit the EN pin voltage to < 4 V.
Constant On-time Modulation
The FAN2315 uses a constant on-time modulation
technique, in which the HS MOSFET is turned on for a
fixed time, set by the modulator, in response to the input
voltage and the frequency setting resistor. This on-time
is proportional to the desired output voltage, divided by
the input voltage. With this proportionality, the frequency
is essentially constant over the load range where
inductor current is continuous.
For buck converter in Continuous-Conduction Mode
(CCM), the switching frequency f_SW is expressed as:
=×
(1)
The on-time generator sets the on-time (t_ON) for the
high-side MOSFET, which results in the switching
frequency of the regulator during steady-state operation.
To maintain a relatively constant switching frequency
over a wide range of input conditions, the input voltage
information is fed into the on-time generator.
t_ON is determined by:
= ×2
(2)
where I_tON is:
=
1
10
×
(3)
where R_FREQ is the frequency-setting resistor
described in the Setting Switching Frequency section;
C_tON is the internal 2.2 pF capacitor; and I_tON is the V_IN
feed-forward current that generates the on-time.
The FAN2315 implements open-circuit detection on the
FREQ pin to protect the output from an infinitely long
on-time. In the event the FREQ pin is left floating,
switching of the regulator is disabled. The FAN2315 is
designed for V_IN input range 4.5 to 15 V, f_SW 200 kHz to
1 MHz, resulting in an I_tON ratio of 1 to 16.
As the ratio of V_OUT to V_IN increases, t_OFF,min introduces a
limit on the maximum switching frequency as calculated
in the following equation, where the factor 1.2 is
included in the denominator to add some headroom for
transient operation:
< _.×
,
,
(4)
Soft-Start (SS)
A conventional soft-start ramp is implemented to provide
a controlled startup sequence of the output voltage. A
current is generated on the SS pin to charge an external
capacitor. The lesser of the voltage on the SS pin and
the reference voltage is used for output regulation.
To reduce V_OUT ripple and achieve a smoother ramp of
the output voltage, t_ON is modulated during soft-start. t_ON
starts at 50% of the steady-state on-time (PWM Mode)
and ramps up to 100% gradually.
During normal operation, the SS voltage is clamped to
400 mV above the FB voltage. The clamp voltage drops
to 40 mV during an overload condition to allow the
converter to recover using the soft-start ramp once the
overload condition is removed. On-time modulation
during SS is disabled when an overload condition exists.
To maintain a monotonic soft-start ramp, the regulator is
forced into PFM Mode during soft-start. The minimum
frequency clamp is disabled during soft-start.
The nominal startup time is programmable through an
internal current source charging the external soft-start
capacitor C_SS:
=×
(5)
where:
C_SS = External soft-start programming capacitor;
I_SS = Internal soft-start charging current source,
10 μA;
t_SS = Soft-start time; and
V_REF = 600 mV
For example; for 1 ms startup time, C_SS=15 nF.
© 2011 Fairchild Semiconductor Corporation
FAN2315 • Rev. 1.0.4
11
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