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参数资料
| 型号： | FEB167 |
| 厂商： | Fairchild Semiconductor |
| 文件页数： | 12/15页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR FAN2106 |
| 标准包装： | 1 |
| 系列： | TinyBuck™ |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，非隔离 |
| 输出电压： | 1.8V |
| 电流 - 输出： | 6A |
| 输入电压： | 8 ~ 20 V |
| 稳压器拓扑结构： | 降压 |
| 频率 - 开关： | 500kHz |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | FAN2106 |
| 相关产品： | FAN2106EMPXDKR-ND - IC REG BUCK SYNC ADJ 6A 25MLP FAN2106EMPXCT-ND - IC REG BUCK SYNC ADJ 6A 25MLP FAN2106EMPXTR-ND - IC REG BUCK SYNC ADJ 6A 25MLP FAN2106MPXDKR-ND - IC REG BUCK SYNC ADJ 6A 25MLP FAN2106MPXCT-ND - IC REG BUCK SYNC ADJ 6A 25MLP FAN2106MPXTR-ND - IC REG BUCK SYNC ADJ 6A 25MLP |
Setting the Ramp Resistor Value
R RAMP resistor plays a critical role in the design by
providing charging current to the internal ramp capacitor
Since the I LIM voltage is set by a 10 μA current source
into the R ILIM resistor, the basic equation for setting the
reference voltage is:
and also serving as a means to provide input voltage
feedforward.
R RAMP is calculated by the following formula:
V RILIM = 10μA*R ILIM
To calculate R ILIM :
(7)
R RAMP ( K Ω ) =
( V IN ? 1 . 8 ) ? V OUT
( 18 ) ? V IN ? f ? 10 ? 6
? 2
where frequency (f) is expressed in KHz.
(5)
R ILIM = V RILIM / 10μA (8)
The voltage V RILIM is made up of two components, V BOT
(which relates to the current through the low-side
MOSFET) and V RMPEAK (which relates to the peak
For wide input operation, first calculate R RAMP for the
minimum and maximum input voltage conditions and
current through the inductor). Combining those two
voltage terms results in:
use larger of the two values calculated.
In all applications, current through the R RAMP pin must
be greater than 10 μA from the equation below for
proper operation:
R ILIM = (V BOT + V RMPEAK )/ 10μA
R ILIM = {0.96 + (I LOAD * R DSON *K T *8)} +
{D*(V IN – 1.8)/(f SW *0.03*R RAMP )}/10μA
(9)
(10)
V IN ? 1 . 8
R RAMP + 2
≥ 10 μ A
(6)
where:
V BOT = 0.96 + (I LOAD * R DSON *K T *8);
If the calculated R RAMP values in Equation (5) result in a
current less than 10 μA, use the R RAMP value that
satisfies Equation (6). In applications with large input
ripple voltage, the R RAMP resistor should be adequately
decoupled from the input voltage to minimize ripple on
the RAMP pin .
Setting the Current Limit
The current limit system involves two comparators. The
MAX I LIMIT comparator is used with a V ILIM fixed-voltage
reference and represents the maximum current limit
allowable. This reference voltage is temperature
compensated to reflect the R DSON variation of the low-
side MOSFET. The ADJUST I LIMIT comparator is used
where the current limit needs to be set lower than the
V ILIM fixed reference. The 10 μA current source does not
track the R DSON changes over temperature, so change is
added into the equations for calculating the ADJUST
I LIMIT comparator reference voltage, as is shown below.
Figure 24 shows a simplified schematic of the over-
current system.
V RMPEAK = D*(V IN – 1.8)/(f SW *0.03*R RAMP );
I LOAD = the desired maximum load current;
R DSON = the nominal R DSON of the low-side MOSFET;
K T = the normalized temperature coefficient for the
low-side MOSFET (on datasheet graph);
D = V OUT /V IN duty cycle;
f SW = Clock frequency in kHz; and
R RAMP = chosen ramp resistor value in k Ω .
After 16 consecutive, pulse-by-pulse, current-limit
cycles, the fault latch is set and the regulator shuts
down. Cycling V CC or EN restores operation after a
normal soft-start cycle (refer to the Auto-Restart
section) .
The over-current protection fault latch is active during
the soft-start cycle. Use 1% resistor for R ILIM .
RAMP
VERR
+
_
+
PWM
COMP
MAX
ILIMIT
PWM
10μA
V CC
VILIM
_
ILIM
+
_
ADJUST
ILIMIT
ILIMTRIP
RILIM
Figure 24. Current-Limit System Schematic
? 2009 Fairchild Semiconductor Corporation
FAN2106 ? Rev. 1.1.2
12
www.fairchildsemi.com
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