FAN2108EMPX Fairchild Semiconductor, FAN2108EMPX Datasheet - Page 10

• Manufacturer Part Number: FAN2108EMPX
• Description: IC REG SYNC BUCK 3-24V 8A 25MLP
• Manufacturer: Fairchild Semiconductor
• Series: TinyBuck™r
• Type: Step-Down (Buck)r

Specifications of FAN2108EMPX

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 19 V
• Current - Output: 8A
• Frequency - Switching: 200kHz ~ 600kHz
• Voltage - Input: 3 ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 25-MLP
• Power - Output: 2.8W
• Mounting Style: SMD/SMT
• Efficiency: 95 %
• Input / Supply Voltage (max): 24 V
• Input / Supply Voltage (min): 3 V
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Operating Temperature Range: - 40 C to + 85 C
• Output Current: 8 A
• Output Power: 2.8 W
• Output Voltage: 3.2 V
• Supply Current: 8 mA
• Switching Frequency: 600 KHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: FAN2108EMPXTR

© 2008 Fairchild Semiconductor Corporation
FAN2108 • Rev. 1.0.1
Circuit Description
Initialization
Once V
HIGH, the IC checks for an open or shorted FB pin
before releasing the internal soft-start ramp (SS).
If R1 is open (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 R
internal SS ramp is not released and the regulator does
not start.
Bias Supply
The FAN2108 requires a 5V supply rail to bias the IC
and provide gate-drive energy. Connect a ≥ 1.0µf X5R
or X7R decoupling capacitor between VCC and PGND.
Since V
supply current is frequency and voltage dependent.
Approximate V
where frequency (f) is expressed in KHz.
Enable
FAN2108 has an internal pull-up to enable pin so that
the IC is enabled once V
small capacitor across EN and AGND delays the rate of
voltage rise on the EN pin. EN pin also serves for the
restart whenever a fault occurs (refer to the Auto-
Restart section). For applications where sequencing is
required, FAN2108 can be enabled (after the V
comes up) with external control, as shown in Figure 20.
Setting the Frequency
Oscillator frequency is determined by an external resistor,
R
Resistance is calculated by:
where R
The regulator cannot start if R
I
R
CC
T
T
, connected between the R(T) pin and AGND.
(
(
K
mA
Figure 20. Enabling with External Control
Ω
)
)
CC
=
=
CC
T
(
is in KΩ and frequency (f) is in KHz.
. 4
10
is used to drive the internal MOSFET gates,
exceeds the UVLO threshold and EN is
58
6
CC
/
+
65
f
[(
)
current (I
V
−
CC
135
227
−
5
+
CC
CC
. 0
) is calculated by:
013
T
is applied. Connecting a
3.3n
is left open.
14
)
•
(
EN
FAN2108
f
−
BIAS
128
is ≤ 1KΩ, the
)]
(2)
CC
(1)
10
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 V
Soft-start time is a function of oscillator frequency.
The regulator does not allow the low-side MOSFET to
operate in full synchronous rectification mode until
internal SS ramp reaches 95% of V
helps the regulator to start on a pre-biased output and
ensures that inductor current does not "ratchet" up
during the soft-start cycle.
V
resets the IC.
Setting the Output Voltage
The output voltage of the regulator can be set from 0.8V
to 80% of V
R
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:
Connect R
R
0
CC
BIAS
BIAS
8 .
UVLO or toggling the EN pin discharges the SS and
V
in Figure 1).
IN
=
EN
FB
SS
Figure 21. Soft-Start Timing Diagram
V
before V
OUT
BIAS
IN
R
between FB and AGND.
−
by an external resistor divider (R1 and
1
0
CC
8 .
1.35V
V
reaches its UVLO threshold.
3200 CLKs
4000 CLKs
+
2400 CLKs
650
1.0V
0.8V
nA
T0.8
T1.0
REF
(~0.76V). This
Fault
Latch
Enable
0.8V
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