參數(shù)資料
型號(hào)： TPS65170RHDT
廠商： TEXAS INSTRUMENTS INC
元件分類： 穩(wěn)壓器
英文描述： 4.2 A SWITCHING REGULATOR, 900 kHz SWITCHING FREQ-MAX, PQCC28
封裝： 5 X 5 MM, GREEN, PLASTIC, QFN-28
文件頁(yè)數(shù)： 14/35頁(yè)
文件大?。?/td> 4698K
代理商： TPS65170RHDT
(
)
(
)
G H(MA X)
S
F
CE
V
= 2
V
2 × V
V
-
FF
1
C
=
2
5 kHz
R
p
(
)
(
)
Q
S
F
GH
P
=
2
V
2
V
I
é
ù
-
-
D
GH
F
P
= I
V
www.ti.com
SLVSA27 – OCTOBER 2009
(19)
Where VS is the output voltage of the boost converter, VF is the forward voltage of each diode and VCE is the
collector-emitter voltage of the PNP transistor (recommended to be at least 1V, to avoid transistor saturation).
Selecting the Feed-Forward Capacitor (Positive Charge Pump)
To improve transient performance, a feed-forward capacitor connected across the upper feedback resistor (R1) is
recommended. The feed-forward capacitor modifies the frequency response of the feedback network by adding
the zero, which improves high frequency gain. For typical applications, a zero at 5kHz is a good place to start, in
which case CFF can be calculated using Equation 20.
(20)
Selecting the PNP Transistor (Positive Charge Pump)
The PNP transistor used to regulate VGH should have a DC gain (hFE) of at least 100 when its collector current is
equal to the charge pump's output current. The transistor should also be able to withstand voltages up to 2×VS
across its collector-emitter (VCE).
The power dissipated in the transistor is given by Equation 21. The transistor must be able to dissipate this
power without its junction becoming too hot. Note that the ability to dissipate power depends heavily on adequate
PCB thermal design.
(21)
Where IGH is the mean (not RMS) output current drawn from the charge pump.
A pull-up resistor is also required between the transistor's base and emitter. The value of this resistor is not
critical, but it should be large enough not to divert significant current away from the base of the transistor. A
value of 100k
Ω is suitable for most applications.
Selecting the Diodes (Positive Charge Pump)
Small-signal diodes can be used for most low current applications (<50mA) and higher rated diodes for higher
power applications. The average current through the diode is equal to the output current, so that the power
dissipated in the diode is given by Equation 22.
(22)
The peak current through the diode occurs during start-up and for a few cycles may be as high as a few amps.
However, this condition typically lasts for <1ms and can be tolerated by many diodes whose repetitive current
rating is much lower. The diodes' reverse voltage rating should be equal to 2×VS.
Table 6. Positive Charge Pump Diode Selection
PART NUMBER
IAVG
IPK
VR
VF
COMPONENT SUPPLIER
BAV99W
150mA
1A for 1ms
75V
1V at 50mA
NXP
BAT54S
200mA
600mA for 1s
30V
0.8V at 100mA
Fairchild Semiconductor
MBR0540
500mA
5.5A for 8ms
40V
0.51 at 500mA
Fairchild Semiconductor
Selecting the Capacitors (Positive Charge Pump)
For lowest output voltage ripple, low-ESR ceramic capacitors are recommended. The actual value is not critical
and 1F to 10F is suitable for most applications. Larger capacitors provide better performance in applications
where large load transient currents are present.
A flying capacitor in the range 100nF to 1F is suitable for most applications. Larger values experience a smaller
voltage drop by the end of each switching cycle, and allow higher output voltages and/or currents to be achieved.
Smaller values tend to be physically smaller and cheaper. For best performance, it is recommended to include a
resistor of a few ohms (2
Ω is a good value to start with) in series with the flying capacitor to limited peak currents
occurring at the instant of switching.
Copyright 2009, Texas Instruments Incorporated
21
Product Folder Link(s) :TPS65170
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