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參數(shù)資料
| 型號(hào)： | MC33680FTB |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類(lèi)： | 穩(wěn)壓器 |
| 英文描述： | Single Output LDO, 400mA, Fixed(2.85V), Low Noise, Fast Transient Response 8-MSOP -40 to 85 |
| 中文描述： | 1.15 A DUAL SWITCHING CONTROLLER, 100 kHz SWITCHING FREQ-MAX, PQFP32 |
| 封裝： | PLASTIC, LQFP-32 |
| 文件頁(yè)數(shù)： | 12/16頁(yè) |
| 文件大?。?/td> | 340K |
| 代理商： | MC33680FTB |
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MC33680
http://onsemi.com
12
DETAILED OPERATING DESCRIPTION (Cont’d)
Synchronous Rectification
A Synchronous Rectifier is used in the main regulator to
enhance efficiency. Synchronous rectifier is normally
realized by powerFET with gate control circuitry which,
however, involved relative complicated timing concerns. In
Figure 19, as main switch M1 is being turned OFF, if the
synchronous switch M2 is just turned ON with M1 not being
completely turned OFF, current will be shunted from the
output bulk capacitor through M2 and M1 to ground. This
power loss lowers overall efficiency. So a certain amount of
dead time is introduced to make sure M1 is completely OFF
before M2 is being turned ON, this timing is indicated as tdh
in Figure 20.
When the main regulator is operating in continuous mode,
as M2 is being turned OFF, and M1 is just turned ON with
M2 not being completed OFF, the above mentioned situation
will occur. So dead time is introduced to make sure M2 is
completed OFF before M1 is being turned ON, this is
indicated as tdl in Figure 20.
When the main regulator is operating in discontinuous
mode, as coil current is dropped to zero, M2 is supposed to
be OFF. Fail to do so, reverse current will flow from the
output bulk capacitor through M2 and then the inductor to
the battery input. It causes damage to the battery. So
M2–voltage–drop sensing comparator (COMP3 of Figure
18) comes with fixed offset voltage to switch M2 OFF
before any reverse current builds up. However, if M2 is
switch OFF too early, large residue coil current flows
through the body diode of M2 and increases conduction loss.
Therefore, determination on the offset voltage is essential
for optimum performance.
Auxiliary Regulator
The Auxiliary Regulator is a boost regulator, applies PFM
scheme to enhance high efficiency and reduce quiescent
current. An internal voltage comparator (COMP1 of Figure
21) detects when the voltage of Pin VAUXFBN drops below
that of Pin VAUXFBP. The internal power BJT is then
switched ON for a fixed–ON–time (or until the internal
current limit is reached), and coil current is allowed to build
up. As the BJT is switched OFF, coil current will flow
through the external Schottky diode to charge up the bulk
capacitor. After a fixed–mimimum–OFF time elapses, next
switching cycle will start if the output of the voltage
comparator is HIGH. Refer to Figure 21, the VAUX
regulation level is determined by the equation as follows,
VAUX
VAUXFBP
1
RAUXb
RAUXa
(V)
Where Max ON Time, TON2, and Min OFF Time, TOFF2
can be determined by the following equations.
TON2
TOFF2
As the Auxiliary Regulator control scheme is the same as
the Main Regulator, equations for conduction mode, Tsw
and Ipk can also be applied, However, to be used for
calculation is referred to Figures 6, 8, or 10.
VBAT
1.7
2.1
10–11
10–12
RIref (S)
RIref (S)
Figure 21. Simplified Block Diagram of Auxiliary Regulator
+ve Edge Delay
R
S
Q
Qb
1–SHOT
for Mn. OFF Time
for Max. ON Time
AGND
ILIM
COMP2
CAUX
33 uF
+
L2
33uH
Auxiliary Regulator
20
18
VBAT
25
21
RAUXb
2200 kOhm
RAUXa
200 kOhm
VAUXFBN
VAUXSW
VAUXBDV
VAUXFBP
COMP1
VCOMP
VAUXEMR
26
senseBJT
Q1
VREF
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