FAN5355 Fairchild Semiconductor, FAN5355 Datasheet - Page 18

• Manufacturer Part Number: FAN5355
• Description: The FAN5355 device is a high-frequency, ultra-fast transient response, synchronous step-down DC-DC converter optimized for low-power applications using small, low-cost inductors and capacitors
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN5355 • Rev. 1.1.0
Table 3. VSEL vs. VOUT
Dec Binary Hex
10 001010 0A
11 001011 0B
12 001100 0C
13 001101 0D
14 001110 0E
15 001111 0F
16 010000 10
17 010001 11
18 010010 12
19 010011 13
20 010100 14
21 010101 15
22 010110 16
23 010111 17
24 011000 18
25 011001 19
26 011010 1A
27 011011 1B
28 011100 1C
29 011101 1D
30 011110 1E
31 011111 1F
32 100000 20
33 100001 21
34 100010 22
35 100011 23
36 100100 24
37 100101 25
38 100110 26
39 100111 27
40 101000 28
41 101001 29
42 101010 2A
43 101011 2B
44 101100 2C
45 101101 2D
46 101110 2E
47 101111 2F
48 110000 30
49 110001 31
50 110010 32
51 110011 33
52 110100 34
53 110101 35
54 110110 36
55 110111 37
56 111000 38
57 111001 39
58 111010 3A
59 111011 3B
60 111100 3C
61 111101 3D
62 111110 3E
63 111111 3F
0
1
2
3
4
5
6
7
8
9
VSEL Value
000000 00
000001 01
000010 02
000011 03
000100 04
000101 05
000110 06
000111 07
001000 08
001001 09
0.7500
0.7625
0.7750
0.7875
0.8000
0.8125
0.8250
0.8375
0.8500
0.8625
0.8750
0.8875
0.9000
0.9125
0.9250
0.9375
0.9500
0.9625
0.9750
0.9875
1.0000
1.0125
1.0250
1.0375
1.0500
1.0625
1.0750
1.0875
1.1000
1.1125
1.1250
1.1375
1.1500
1.1625
1.1750
1.1875
1.2000
1.2125
1.2250
1.2375
1.2500
1.2625
1.2750
1.2875
1.3000
1.3125
1.3250
1.3375
1.3500
1.3625
1.3750
1.3875
1.4000
1.4125
1.4250
1.4375
1.4500
1.4625
1.4750
1.4875
1.5000
1.5125
1.5250
1.5375
00, 03
0.7500
0.7625
0.7750
0.7875
0.8000
0.8125
0.8250
0.8375
0.8500
0.8625
0.8750
0.8875
0.9000
0.9125
0.9250
0.9375
0.9500
0.9625
0.9750
0.9875
1.0000
1.0125
1.0250
1.0375
1.0500
1.0625
1.0750
1.0875
1.1000
1.1125
1.1250
1.1375
1.1500
1.1625
1.1750
1.1875
1.2000
1.2125
1.2250
1.2375
1.2500
1.2625
1.2750
1.2875
1.3000
1.3125
1.3250
1.3375
1.3500
1.3625
1.3750
1.3875
1.4000
1.4125
1.4250
1.4375
1.4375
1.4375
1.4375
1.4375
1.4375
1.4375
1.4375
1.4375
VOUT
02, 08
1.1875
1.2000
1.2125
1.2250
1.2375
1.2500
1.2625
1.2750
1.2875
1.3000
1.3125
1.3250
1.3375
1.3500
1.3625
1.3750
1.3875
1.4000
1.4125
1.4250
1.4375
1.4500
1.4625
1.4750
1.4875
1.5000
1.5125
1.5250
1.5375
1.5500
1.5625
1.5750
1.5875
1.6000
1.6125
1.6250
1.6375
1.6500
1.6625
1.6750
1.6875
1.7000
1.7125
1.7250
1.7375
1.7500
1.7625
1.7750
1.7875
1.8000
1.8125
1.8250
1.8375
1.8500
1.8625
1.8750
1.8875
1.9000
1.9125
1.9250
1.9375
1.9500
1.9625
1.9750
06
18
Software Enable
The EN_DCDC bit, VSELx[7] can enable the regulator in
conjunction with the EN pin. Setting EN_DCDC with EN HIGH
begins the soft-start sequence described above.
Table 4. EN_DCDC Behavior
Light-Load (PFM) Operation
The FAN5355 offers a low-ripple, single-pulse PFM mode to
save power and improve efficiency when the load current is
very low. PFM operation features:



PFM begins after the inductor current has become
discontinuous, crossing zero during the PWM cycle in 32
consecutive cycles. PFM exit occurs when discontinuous
current mode (DCM) operation cannot supply sufficient current
to maintain regulation. During PFM mode, the inductor current
ripple is about 40% higher than in PWM mode. The load
current required to exit PFM mode is thereby about 20%
higher than the load current required to enter PFM mode,
providing sufficient hysteresis to prevent “mode chatter.”
While PWM ripple voltage is typically less than 4mV
ripple voltage can be up to 30mV
prevent significant undershoot when a load transient occurs,
the initial DC set point for the regulator in PFM mode is set
10mV higher than in PWM mode. This offset decays to about
5mV after the regulator has been in PFM mode for ~100s.
The maximum instantaneous voltage in PFM is 30mV above
the set point.
PFM mode can be disabled by writing to the mode control bits:
CONTROL1[3:0] (see Table 10 for details).
Some vendors provide both “Light PFM” (LPFM) and “Fast
PFM” (FPFM) modes, while the FAN5355 provides only one
PFM mode. The FAN5355’s single PFM mode features the
fast transient recovery of FPFM, but does this with the low
quiescent current consumption similar to LPFM mode.
EN_DCDC Bit
Smooth transitions between PFM and PWM modes
Single-pulse operation for low ripple
Predictable PFM entry and exit currents.
0
1
1
0
EN Pin
0
1
0
1
PP
OFF
OFF
I
ON
ON
during very light load. To
2
C
www.fairchildsemi.com
REGULATOR
OFF
OFF
OFF
ON
PP
, PFM