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参数资料
| 型号： | LT1956-5IGN |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 570 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, PLASTIC, SSOP-16 |
| 文件页数： | 6/24页 |
| 文件大小： | 437K |
| 代理商： | LT1956-5IGN |
14
LT1956/LT1956-5
CATCH DIODE
Highest efficiency operation requires the use of a Schottky
type diode. DC switching losses are minimized due to its
low forward voltage drop, and AC behavior is benign due
to its lack of a significant reverse recovery time. Schottky
diodes are generally available with reverse voltage ratings
of 60V and even 100V, and are price competitive with other
types.
The use of so-called “ultrafast” recovery diodes is gener-
ally not recommended. When operating in continuous
mode, the reverse recovery time exhibited by “ultrafast”
diodes will result in a slingshot type effect. The power
internal switch will ramp up VIN current into the diode in an
attempt to get it to recover. Then, when the diode has
finally turned off, some tens of nanoseconds later, the VSW
node voltage ramps up at an extremely high dV/dt, per-
haps 5 to even 10V/ns ! With real world lead inductances,
the VSW node can easily overshoot the VIN rail. This can
result in poor RFI behavior and if the overshoot is severe
enough, damage the IC itself.
The suggested catch diode (D1) is an International Recti-
fier 10MQ060N Schottky. It is rated at 1.5A average
forward current and 60V reverse voltage. Typical forward
voltage is 0.63V at 1.5A. The diode conducts current only
during switch off time. Peak reverse voltage is equal to
regulator input voltage. Average forward current in normal
operation can be calculated from:
ID(AVG) = IOUT (1 – DC)
This formula will not yield values higher than 1.5A with
maximum load current of 1.5A. The only reason to
consider a larger diode is the worst-case condition of a
high input voltage and shorted output. With a shorted
condition, diode current will increase to a typical value of
2A, determined by peak switch current limit. This is safe
for short periods of time, but it would be prudent to check
with the diode manufacturer if continuous operation
under these conditions must be tolerated.
BOOST PIN
For most applications, the boost components are a 0.1
F
capacitor and a 1N914 diode. The anode is typically
connected to the regulated output voltage to generate a
voltage approximately VOUT above VIN to drive the output
stage. However, the output stage discharges the boost
capacitor during the on time of the switch. The output
driver requires at least 3V of headroom throughout this
period to keep the switch fully saturated. If the output
voltage is less than 3V, it is recommended that an alternate
boost supply is used. The boost diode can be connected to
the input, although, care must be taken to prevent the 2
×
VIN boost voltage from exceeding the BOOST pin absolute
maximum rating. The additional voltage across the switch
driver also increases power loss, reducing efficiency. If
available, an independent supply can be used with a local
bypass capacitor.
A 0.1
F boost capacitor is recommended for most appli-
cations. Almost any type of film or ceramic capacitor is
suitable, but the ESR should be <1
to ensure it can be
fully recharged during the off time of the switch. The
capacitor value is derived from worst-case conditions of
1800ns on time, 42mA boost current and 0.7V discharge
ripple. The boost capacitor value could be reduced under
less demanding conditions, but this will not improve
circuit operation or efficiency. Under low input voltage and
low load conditions, a higher value capacitor will reduce
discharge ripple and improve start-up operation.
SHUTDOWN FUNCTION AND UNDERVOLTAGE
LOCKOUT
Figure 4 shows how to add undervoltage lockout (UVLO)
to the LT1956. Typically, UVLO is used in situations where
the input supply is
current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
Threshold voltage for lockout is about 2.38V. A 5.5
A bias
current flows
out of the pin at this threshold. The internally
generated current is used to force a default high state on
the shutdown pin if the pin is left open. When low shut-
down current is not an issue, the error due to this current
can be minimized by making RLO 10k or less. If shutdown
APPLICATIO S I FOR ATIO
WU
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