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參數資料
| 型號： | LM3477A |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | High Efficiency High-Side N-Channel Controller for Switching Regulator |
| 中文描述： | 高效率高側N溝道開關穩(wěn)壓器控制器 |
| 文件頁數： | 14/23頁 |
| 文件大小： | 815K |
| 代理商： | LM3477A |
Design Section
(Continued)
This is the fraction of the switching period that the switch is
on. The switch is off for the remainder of the period. This
fraction is expressed as:
D’ = 1 D
The LM3477/A has limits for the maximum and minimum
duty cycle (see
ELECTRICAL CHARACHTERISTICS
). The
maximum duty cycle of 93% (typical) will limit how low the
input voltage may drop while maintaining a regulated output
voltage (the dropout voltage). In situations where a very low
dropout voltage is required, it is necessary to include V
, V
Q
and V
losses in the maximum duty cycle calculation.
Voltage drops in the inductor will lower the dropout voltage
as well.
The minimum duty cycle of the LM3477/Acorresponds to the
minimum on time, or blank out time (see
ELECTRICAL
CHARACHTERISTICS
).
D
MIN
= T
MIN
*
f
s
This will not limit how high the input voltage can rise, how-
ever the LM3477/A will operate in hysteretic mode once the
operating duty cycle decreases to the minimum duty cycle.
Programming the Current Limit/Hysteretic Threshold
DEFINITIONS:
Current Limit: The current limit is the point at which the
LM3477/A begins to limit the peak switch current. The cur-
rent limit in the LM3477/A varies with duty cycle, which is a
function of the V
IN
V
OUT
differential.
Hysteric Threshold: Hysteretic threshold is the current at
which the LM3477/A enters the hysteretic mode of operation
(see
OVER VOLTAGE PROTECTION
section). The hyster-
etic threshold is with respect to the peak switch current.
Setting Current Limit and Hysteretic Threshold
The adjustable current limit of the LM3477/A is set by the
sense resistor R
SN
. The voltage across R
SN
is compared to
an internal control voltage V
C
. The onset of current limiting is
when V
equals V
, or V
. V
SEN
is defined
here as the differential voltage from the V
pin. V
decreases as the duty cycle increases, as shown in
Figure 7
. Therefore, it is important to know both V
and V
at the maximum operating duty cycle, or lowest
V
IN
condition.
V
CL(MIN)
= V
CL(0)(MIN)
D
(MAX)
(V
CL(0)(MIN)
V
CL(100)(MIN)
)
where D
MAX
is the duty cycle at the lowest V
IN
condition.
To avoid current limit,
V
SEN(peak)
<
V
CL(MIN)
Therefore,
200033K3
FIGURE 7. Current Limit and Hysteretic Threshold vs
Duty Cycle
L
www.national.com
14
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