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)： 16/35頁(yè)
文件大?。?/td> 4698K
代理商： TPS65170RHDT
1
GL(SC)
2
R
V
=
1.65 V
1
R
-
-
÷
è
1
GL
LO GIC
2
R
V
=
V
R
-
GL
1
2
LOGIC
V
R
= R
V
(
)
GL(MAX)
IN
F
CE
V
=
V + 2
V
+ V
-
(
)
Q
IN
F
GL
P
=
V
2
V
I
é
ù
-
-
D
GL
F
IP
= I
V
www.ti.com
SLVSA27 – OCTOBER 2009
To ensure proper start-up under normal conditions, circuit designers should ensure that the full load current is not
drawn by the load until the feedback voltage VFBN is below the short circuit threshold voltage. The value of VGL
beyond which the negative charge pump no longer works in short-circuit mode is given by Equation 23.
(23)
NEGATIVE CHARGE PUMP DESIGN PROCEDURE
Setting the Output Voltage (Negative Charge Pump)
The negative charge pump's output voltage is programmed by a resistor divider according to Equation 24.
(24)
Rearranging Equation 25, the values of R1 and R2 can be easily calculated.
(25)
A current of the order of 1mA through the resistor network ensures good accuracy and increases the circuit's
immunity to noise. It also ensures a minimum load on the charge pump, which reduces output voltage ripple
under no-load conditions. A good approach is to assume a value of about 3.3k for the lower resistor (R2) and
then select the upper resistor (R1) to set the desired output voltage.
Note that the maximum voltage in an application is determined by the boost converter's output voltage and the
voltage drop across the diodes and NPN transistor. For a typical application in which the negative charge pump
is configured as a voltage inverter, the maximum (i.e., most negative) output voltage is given by Equation 26.
(26)
Where VF is the forward voltage of each diode and VCE is the collector-emitter voltage of the NPN transistor
(recommended to be at least 1V, to avoid transistor saturation).
Selecting the NPN Transistor (Negative Charge Pump)
The NPN transistor used to regulate VGL 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 VIN
across its collector-emitter (VCE).
The power dissipated in the transistor is given by Equation 27. The transistor must be able to dissipate this
power without its junction becoming too hot. Note that the ability to dissipate power depends on adequate PCB
thermal design.
(27)
Where IGL is the mean (not RMS) output current drawn from the charge pump.
Selecting the Diodes (Negative 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 28.
(28)
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 at least 2×VIN.
Copyright 2009, Texas Instruments Incorporated
23
Product Folder Link(s) :TPS65170
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