MAX16928_12 Datasheet, PDF (13/20 Pages) Maxim Integrated Products – Automotive TFT-LCD Power Supply with Boost Converter and Gate Voltage Regulators

MAX16928
Automotive TFT-LCD Power Supply with Boost
Converter and Gate Voltage Regulators
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
55FA 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.
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
satisfied when I_DR x (h_FE + 1) is greater than the maxi-
mum 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 0.1FF ceramic capacitor. For applications in which
the boost output current exceeds 300mA, connect a
30kI resistor from DR to ground.
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 Operating 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.
The number of positive charge-pump stages needed is
given by:
n_CP
=
V_GH+V_DROPOUT − V_SH
V_SH+V_SCHOTTKY − 2 × V_D
and the number of negative charge-pump stages is
given by:
n
CN
=
V_SH
|V_GL |+V_DROPOUT
+ V_SCHOTTKY − 2
×
V_D
where n_CP is the number of positive charge-pump stag-
es, n_CN is the number of negative charge-pump stages,
V_GH is the positive-gate voltage regulator output volt-
age, V_GL is the negative-gate voltage regulator output
voltage, V_SH is the boost converter’s output voltage, V_D
VSH
VCP
VCN
LXP
LXP
Figure 2. Multistage Charge Pump for Positive Output Voltage
Figure 3. Multistage Charge Pump for Negative Output Voltage
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