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FAN2103EMPX Datasheet(PDF) 10 Page - Fairchild Semiconductor

No. de pieza FAN2103EMPX
Descripción Electrónicos  TinyBuck??3A, 24V Input, Integrated Synchronous Buck Regulator
Download  14 Pages
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Fabricante Electrónico  FAIRCHILD [Fairchild Semiconductor]
Página de inicio  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN2103EMPX Datasheet(HTML) 10 Page - Fairchild Semiconductor

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© 2007 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FAN2103 Rev. 1.0.5
10
Circuit Description
Initialization
Once VCC exceeds the UVLO threshold and EN is
HIGH, the IC checks for an open or shorted FB pin
before releasing the internal soft-start ramp (SS).
If R1 is open (as shown in Figure 1), the error amplifier
output (COMP) is forced LOW and no pulses are
generated. After the SS ramp times out (T1.0), an under-
voltage latched fault occurs.
If the parallel combination of R1 and RBIAS is ≤ 1KΩ, the
internal SS ramp is not released and the regulator does
not start.
Soft-Start
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), the “Fault Latch” is inhibited.
To avoid skipping the soft-start cycle, it is necessary to
apply VIN before VCC reaches its UVLO threshold.
Soft-start time is a function of oscillator frequency.
SS
1.35V
FB
EN
0.8V
T0.8
T1.0
3200 CLKs
4000 CLKs
Fault
Latch
Enable
0.8V
1.0V
2400 CLKs
Figure 21. Soft-Start Timing Diagram
The regulator does not allow the low-side MOSFET to
operate in full synchronous rectification mode until
internal SS ramp reaches 95% of VREF (~0.76V). This
helps the regulator start against pre-biased outputs (as
shown in Figure 16) and ensures that inductor current
does not "ratchet" up during the soft-start cycle.
VCC UVLO or toggling the EN pin discharges the SS and
resets the IC.
Bias Supply
The FAN2103 requires a 5V supply rail to bias the IC
and provide gate-drive energy and controller power.
Connect a
≥ 1.0µf X5R or X7R decoupling capacitor
between VCC and PGND. Whenever the EN pin is
pulled up to VCC, the 5V supply connected to VCC should
be turned ON after VIN comes up. If the power supply is
turned ON using EN pin with an external control after
VCC and VIN come up, the VCC and VIN power
sequencing is not relevant.
Since VCC is used to drive the internal MOSFET gates,
supply current is frequency and voltage dependent.
Approximate VCC current (ICC) can be calculated using:
)]
128
f
(
)
013
.
0
227
5
V
[(
58
.
4
I
CC
)
mA
(
CC
+
+
=
(1)
where frequency (f) is expressed in KHz.
Setting the Output Voltage
The output voltage of the regulator can be set from 0.8V
to ~80% of VIN by an external resistor divider (R1 and
RBIAS in Figure 1).
The internal reference is 0.8V with 650nA, sourced from
the FB pin to ensure that if the pin is open, the regulator
does not start.
The external resistor divider is calculated using:
nA
650
1
R
V
8
.
0
V
R
V
8
.
0
OUT
BIAS
+
=
(2)
Connect RBIAS between FB and AGND.
Setting the Frequency
Oscillator frequency is determined by an external resistor,
RT, connected between the R(T) pin and AGND:
135
)
R
65
(
10
f
T
6
)
KHz
(
+
=
(3)
where RT is expressed in KΩ.
65
135
)
f
/
10
(
R
6
)
K
(
T
=
Ω
(4)
where frequency (f) is expressed in KHz.
The regulator does not start if RT is left open.


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