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参数资料
| 型号： | FAN4855MTCX |
| 厂商： | Fairchild Semiconductor |
| 文件页数： | 8/12页 |
| 文件大小： | 0K |
| 描述： | IC REG BST SYNC ADJ 0.5A 8TSSOP |
| 产品变化通告： | Product Discontinuation 03/Dec/2009 |
| 标准包装： | 2,500 |
| 类型： | 升压（升压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 3 V ~ 5 V |
| 输入电压： | 1.6 V ~ 4.5 V |
| 电流 - 输出： | 500mA |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-TSSOP（0.173"，4.40mm 宽） |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 8-TSSOP |
Block Diagram
LBO
4
7
V L
SHDN
Control
Logic
2
LBI
3
–
A3
0.39V
+
I LIMIT
Q 2
SHDN
V IN
1
V OUT
Start-Up
Synchronous
Rectifier
V OUT
Control
+
6
A2
–
Minimum
Off-Time
Current
Logic
Limit
Control
I LIMIT
V FB
5
I LIMIT SHDN
Variable
On-Time
One Shot
Q 1
N
1
A1
–
+
V REF
8
GND
I L ( PEAK ) = --------------------------
Functional Description
Boost Regulator
FAN4855 is an adjustable boost regulator that combines
variable ON and minimum OFF architecture with syn-
chronous recti?cation. Unique control circuitry provides
high-ef?ciency power conversion for both light and heavy
loads by transitioning between discontinuous and contin-
uous conduction mode based on load conditions. There
is no oscillator; a constant-peak-current limit of 0.8A in
the inductor allows the inductor current to vary between
this peak limit and some lesser value. The switching fre-
quency depends upon the load, the input and output volt-
age ranging up to 430kHz.
The input voltage V IN comes to V IN pin and through the
external inductor to the V L pin of the device. The loop
from V OUT closes through the external resistive voltage
divider to the feedback pin V FB . The transfer ratio of this
divider determines the output voltage. When V FB voltage
drops below the V REF = 1.24V the error ampli?er A1 sig-
nals to regulator to deliver charge to the output by trig-
gering the Variable On-Time One Shot. One Shot
generates a pulse at the gate of the Power NMOS tran-
sistor Q1. This transistor will charge the Inductor L1 for
the time interval T ON resulting in a peak current given by:
T ON × V IN
L 1
FAN4855 Rev. 1.0.1
8
When the one–shot times out, the Q1 transistor releases
the V L pin, allowing the inductor to ?y-back and momen-
tarly charge the output through the body diode of the
transistor Q2. But, as the voltage across the Q2 changes
polarity, its gate will be driven low by the Synchronous
Recti?er Control Circuit (SRC), causing Q2 to short out
its body diode. The inductor then delivers the charge to
the load by discharging into it through Q2.
Under light load conditions, the amount of energy deliv-
ered in this single pulse satis?es the voltage-control loop,
and the converter does not command any more energy
pulses until the output drops again below the lower-volt-
age threshold. Under medium and heavy loads, a single
energy pulse is not suf?cient to force the output voltage
above its upper threshold before the minimum off time
has expired and a second charge cycle is commanded.
Since the inductor current has not reached zero in this
case, the peak current is greater than the previous value
at the end of the second cycle. The result is a ratcheting
of inductor current until either the output voltage is satis-
?ed, or the converter reaches its set current limit.
After a period of time T OFF > 1μS, determined by Mini-
mum Off–Time Logic and if V OUT is low (V FB < V REF ), the
Variable On-Time One Shot will be turned ON again and
the process repeats.
The output capacitor of the converter ?lters the variable
component, limiting the output voltage ripple to a value
determined by its capacitance and its ESR.
www.fairchildsemi.com
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