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MAX16929
Automotive TFT-LCD Power Supply with Boost
Converter and Gate Voltage Regulators
Table 3. Compensation Component Values
V SH (V) 8 18
I SH (mA) 200 200
V INA (V) 3.3 5
55 F A and the resulting gate source voltage (V GS ) turns
on the FET. When the gate drive is removed under a fault
condition or in shutdown, R SG bleeds off charge to turn
off the FET. Size R SG to produce the V GS needed to turn
on the FET.
P IN (W)
L (μH)
R COMPV (k I )
C COMPV (pF)
C COMPI (pF)
V SH
LXP
1.75
5
33
220
820
3.75
5
39
180
330
V CP
1.8V/3.3V Regulator Controller
npn Bipolar Transistor Selection
There are two important considerations in selecting the
pass npn bipolar transistor: current gain (h FE ) and power
dissipation. Select a transistor with an h FE high enough to
ensure adequate drive capability. This condition is satis-
fied when I DR x (h FE + 1) is greater than the maximum
load current. The regulator can source I DR = 4.5mA (min).
The transistor should be capable of dissipating:
P NPN_REG = (V INA - V REG_OUT ) × I LOAD(MAX)
where V REG_OUT = 1.8V or 3.3V. Bypass DR to ground
with a 0.1 F F ceramic capacitor. For applications in which
the boost output current exceeds 300mA, connect a
30k I resistor from DR to ground.
Figure 2. Multistage Charge Pump for Positive Output Voltage
V CN
LXP
Figure 3. Multistage Charge Pump for Negative Output Voltage
To further optimize transient response, vary R COMPV
in 20% steps and C COMPV in 50% steps while observ-
ing transient-response waveforms. The ideal transient
response is achieved when the output settles quickly with
little or no overshoot. Connect the compensation network
to the analog ground plane.
Use the following formula to calculate the value for C COMPI :
C COMPI ≤ 950 × 10 -6 × L/(V SH + V SCHOTTKY - V INA )
p-Channel FET Selection
The p-channel FET used to gate the boost converter’s
input should have low on-resistance. Connect a resistor
(R SG ) between the source and gate of the FET. Under
normal operation, R SG carries a gate drive current of
Supply Considerations
INA needs to be at least 4.5V for the 3.3V regulator to
operate properly.
Charge Pumps
Selecting the Number of Charge-Pump Stages
For most applications, a single charge-pump stage is
sufficient, as shown in the Typical Application Circuit .
Connect the flying capacitors to LXP. The output voltages
generated on the storage capacitors are given by:
V CP = 2 x V SH + V SCHOTTKY - 2 x V D
V CN = -(V SH + V SCHOTTKY - 2 x V D )
where V CP is the positive supply for the positive-gate volt-
age regulator, and V CN is the negative supply for the neg-
ative-gate voltage regulator. Where larger output voltages
are needed, use multistage charge pumps (however, the
maximum charge-pump voltage is limited by the absolute
maximum ratings of CP and DRVN). Figure 2 and Figure 3
show the configuration of a multistage charge pump for
both positive and negative output voltages.
For mutistage charge pumps the output voltages are:
V CP = V SH + n × (V SH + V SCHOTTKY - 2 x V D )
V CN = -n × (V SH + V SCHOTTKY - 2 x V D )
For highest efficiency, choose the lowest number of
charge-pump stages that meets the output requirement.
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