2 darlington bipolar, 3 enhancement mode fe, 3 enhancement mode fet as the pass device -11 – Maxim Integrated MAXQ7667 User Manual
Page 277: Maxq7667 user’s guide
16.3.1.2 Darlington Bipolar Transistor as the Pass Device
Bipolar transistors may also be used for the pass transistor. In Figure 16-5 a Darlington pair is used to minimize the amount of base
current that is required. Pullup resistor R1 is required to start the circuit and as part of the level shifter. All the base current for Q1 comes
from R1. R1 must be sized so that it can supply the required base current when V
IN
is at its minimum and Q1 has the lowest beta.
Excess current from R1 flows through the level-shifting diodes and into the error amp. This excess current, which will be greatest when
V
IN
is at its maximum and Q1 has a high beta, must not exceed the current sink capability of the error amp (500µA).
Diodes D1 to D4 are required for level shifting the control voltage. To a first approximation, the voltage across D1 and D2 will match
the base-emitter voltage of the Darlington. This makes the voltage at the cathode of D2 roughly equal to DVDDIO. Adding a third diode
places the output of the error amp one diode drop, or about 600mV, below DVDDIO. While this voltage is acceptable, it is a condition
that exists at room temperature. At elevated temperatures the diode voltage is reduced and the voltage available at the base of the
transistor becomes marginal. For a robust design it is best to use a minimum of four level-shifting diodes with a Darlington transistor.
The dropout voltage for a bipolar pass transistor is the sum of the base-emitter voltage(s) and the voltage required across R1 to sup-
ply the necessary base current.
16.3.1.3 Enhancement Mode FET as the Pass Device
An enhancement mode FET may also be used for a pass transistor. R1 biases the gate during startup and then becomes part of the
level-shifting circuit. The maximum V
IN
and the current sinking capability of the error amp determine the minimum value of R1. The max-
imum value for R1 should be limited to 100k
Ω for stability reasons. R1 values between 25kΩ and 100kΩ work in nearly all applications.
The number of diodes in the level-shifting string varies with the choice of FET and the temperature range. At all temperatures
(V
GS(TH)
- N V
F
) and (V
GS(TH)
+ I/S - N V
F
) must both be between -0.1V and -3.0V, where V
F
is the forward voltage of the diode, N is
the number of diodes, V
GS(TH)
is the threshold voltage of the FET, S is the transconductance, and I is the supply current for the
MAXQ7667 and any other devices powered from DVDDIO.
16-11
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MAXQ7667 User’s Guide
Figure 16-5. DVDDIO Using a Darlington Pass Transistor
MAXQ7667
R2
10
Ω
ERROR
AMP
VOLTAGE
REFERENCE
DVDDIO
V
IN
5V
Q1
C1
0.1
µF
GATE5
C2
0.1
µF
R1
D4
D3
D2
D1