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参数资料
| 型号： | LM5009SDX |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 150 mA, 100V Step-Down Switching Regulator |
| 中文描述： | 0.37 A SWITCHING REGULATOR, DSO8 |
| 封装： | 4 X 4 MM, EXPOSED PAD, LLP-8 |
| 文件页数： | 14/14页 |
| 文件大小： | 647K |
| 代理商： | LM5009SDX |
ON-Time Generator and Shutdown (Continued)
Current Limit
The LM5009 contains an intelligent current limit OFF timer. If
the current in the Buck switch exceeds 0.31A the present
cycle is immediately terminated, and a non-resetable OFF
timer is initiated. The length of off-time is controlled by an
external resistor (R
CL) and the FB voltage. When FB = 0V, a
maximum off-time is required, and the time is preset to 35s.
This condition occurs when the output is shorted, and during
the initial part of start-up. This amount of time ensures safe
short circuit operation up to the maximum input voltage of
95V. In cases of overload where the FB voltage is above
zero volts (not a short circuit) the current limit off-time will be
less than 35s. Reducing the off-time during less severe
overloads reduces the amount of foldback, recovery time,
and the start-up time. The off-time is calculated from the
following equation:
T
OFF =10
-5 / (0.285 + (V
FB /6.35x10
-6 xR
CL))
(3)
The current limit sensing circuit is blanked for the first 50-
70ns of each on-time so it is not falsely tripped by the current
surge which occurs at turn-on. The current surge is required
by the re-circulating diode (D1) for its turn-off recovery.
N - Channel Buck Switch and
Driver
The LM5009 integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap ca-
pacitor and an internal high voltage diode. A 0.01F ceramic
capacitor (C4) connected between the BST pin and SW pin
provides the voltage to the driver during the on-time.
During each off-time, the SW pin is at approximately -1V, and
the bootstrap capacitor charges from Vcc through the inter-
nal diode. The minimum OFF timer ensures a minimum time
each cycle to recharge the bootstrap capacitor.
An external re-circulating diode (D1) carries the inductor
current after the internal buck switch turns off. This diode
should be of the Ultra-fast or Schottky type to minimize
turn-on losses and current over-shoot.
Thermal Protection
The LM5009 should be operated so the junction temperature
does not exceed 125C during normal operation. An internal
Thermal Shutdown circuit is provided to protect the LM5009
in the event of a higher than normal junction temperature.
When activated, typically at 165C, the controller is forced
into a low power reset state, disabling the buck switch. This
feature prevents catastrophic failures from accidental device
overheating. When the junction temperature reduces below
140C (typical hysteresis = 25C), the buck switch is en-
abled, and normal operation is resumed.
Applications Information
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values will be illus-
trated with a design example. Refer to Figure 1. The follow-
ing steps will configure the LM5009 for:
Input voltage range (Vin): 12V to 90V
Output voltage (V
OUT1): 10V
Load current (for continuous conduction mode): 100mA
to 150mA
R1 and R2: From Figure 1,V
OUT1 =VFB x (R1 + R2) / R2,
and since V
FB = 2.5V, the ratio of R1 to R2 calculates as 3:1.
Standard values of 3.01 k
(R1) and 1.00 k (R2) are
chosen. Other values could be used as long as the 3:1 ratio
is maintained. The selected values, however, provide a small
amount of output loading (2.5 mA) in the event the main load
is disconnected. This allows the circuit to maintain regulation
until the main load is reconnected.
F
s and RON: Unless the application requires a specific fre-
quency, the choice of frequency is generally a compromise
since it affects the size of L1 and C2, and the switching
losses. The maximum allowed frequency, based on a mini-
mum on-time of 250 ns, is calculated from:
F
MAX =VOUT /(VINMAX x 250 ns)
For this exercise, Fmax = 444 kHz. From equation 1, R
ON
calculates to 180 k
. A standard value 237 k resistor will
be used to allow for tolerances in equation 1, resulting in a
nominal frequency of 337 kHz.
L1: The main parameter affected by the inductor is the
output current ripple amplitude. The choice of inductor value
therefore depends on both the minimum and maximum load
currents, keeping in mind that the maximum ripple current
occurs at maximum Vin.
a) Minimum load current: To maintain continuous conduc-
tion at minimum Io (100 mA), the ripple amplitude (I
OR) must
20165807
FIGURE 4. Shutdown Implementation
LM5009
www.national.com
9
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