參數(shù)資料
型號(hào)： ADM3311EARS-REEL7
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 6/24頁(yè)
文件大?。?/td> 0K
描述： IC TXRX RS-232 3:5 2.7V 28SSOP
標(biāo)準(zhǔn)包裝： 500
類型： 收發(fā)器
驅(qū)動(dòng)器/接收器數(shù)： 3/5
規(guī)程： RS232
電源電壓： 2.7 V ~ 3.6 V
安裝類型： 表面貼裝
封裝/外殼： 28-SSOP（0.209"，5.30mm 寬）
供應(yīng)商設(shè)備封裝： 28-SSOP
包裝： 帶卷 (TR)
ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E
Data Sheet
Rev. I | Page 14 of 24
A block diagram of the Green Idle circuit is shown in Figure 26.
Both V+ and V are monitored and compared to a reference
voltage derived from an on-chip band gap device. If either V+
or V fall below 7 V1, the oscillator starts up until the voltage
rises above 7.25 V2.
TRANSCEIVERS
START/STOP
V+
V–
SHUTDOWN
CHARGE
PUMP
V– VOLTAGE
COMPARATOR
WITH 250mV
HYSTERESIS
BAND GAP
VOLTAGE
REFERENCE
V+ VOLTAGE
COMPARATOR
WITH 250mV
HYSTERESIS
02
91
5-
0
26
Figure 26. Block Diagram of Green Idle Circuit
The operation of Green Idle for V+ under various load
conditions is illustrated in Figure 27. Under light load
conditions, C1 is maintained in a charged condition, and only a
single oscillator pulse is required to charge up C2. Under these
conditions, V+ may actually overshoot 7.25 V2 slightly.
OSC
V+
OVERSHOOT
LIGHT
LOAD
7.25V1
7V2
OSC
V+
7.25V1
7V2
OSC
V+
7.25V1
7V2
MEDIUM
LOAD
HEAVY
LOAD
1FOR ADM3310E AND ADM3315E REPLACE WITH 6.5V.
2FOR ADM3310E AND ADM3315E REPLACE WITH 6.25V.
0291
5-
027
Figure 27. Operation of Green Idle under Various Load Conditions
Under medium load conditions, it may take several cycles for
C2 to charge up to 7.25 V2. The average frequency of the
oscillator is higher because there are more pulses in each burst
and the bursts of pulses are closer together and more frequent.
Under high load conditions, the oscillator is on continuously if
the charge pump output cannot reach 7.25 V2.
Green Idle Vs. Shutdown
Shutdown mode minimizes power consumption by shutting
down the charge pump altogether. In this mode, the switches in
the voltage tripler are configured so V+ is connected directly to
VCC. V is zero because there is no charge pump operation to
charge C5. This means there is a delay when coming out of
shutdown mode before V+ and V achieve their normal
operating voltages. Green Idle maintains the transmitter supply
voltages under transmitter idle conditions so this delay does not
occur.
Doesn’t Green Idle Increase Supply Voltage Ripple?
The ripple on the output voltage of a charge pump operating in
open-loop depends on three factors: the oscillator frequency,
the value of the reservoir capacitor, and the load current. The
value of the reservoir capacitor is fixed. Increasing the oscillator
frequency decreases the ripple voltage; decreasing the oscillator
frequency increases it. Increasing the load current increases the
ripple voltage; decreasing the load current decreases it. The
ripple voltage at light loads is naturally lower than that for high
load currents.
Using Green Idle, the ripple voltage is determined by the high
and low thresholds of the Green Idle circuit. These are
nominally 7 V1 and 7.25 V2, so the ripple is 250 mV under most
load conditions. With very light load conditions, there may be
some overshoot above 7.25 V2, so the ripple is slightly greater.
Under heavy load conditions where the output never reaches
7.25 V2, the Green Idle circuit is inoperative and the ripple
voltage is determined by the load current, the same as in a
normal charge pump.
What about Electromagnetic Compatibility?
Green Idle does not operate with a constant oscillator
frequency. As a result, the frequency and spectrum of the
oscillator signal vary with load. Any radiated and conducted
emissions also vary accordingly. Like other Analog Devices
RS-232 transceiver products, the ADM33xxE devices feature
slew rate limiting and other techniques to minimize radiated
and conducted emissions.
1 For ADM3310E and ADM3315E, replace with 6.5 V.
2 For ADM3310E and ADM3315E, replace with 6.25 V.
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