Oscillator frequency (freq), Fault protection, Digital soft-start – Rainbow Electronics MAX1543 User Manual
Page 13
MAX1542/MAX1543
TFT LCD DC-to-DC Converter with
Operational Amplifiers
______________________________________________________________________________________
13
(2.5V rising, 2.35V falling, typ) to ensure that the input
voltage is high enough for reliable operation. The
150mV (typ) hysteresis prevents supply transients from
causing a restart. Once the input voltage exceeds the
UVLO threshold, startup begins. When the input volt-
age falls below the UVLO threshold, the controller turns
off the N-channel MOSFET, the switch control block
turns off Q1, and the operational amplifier outputs float.
For the MAX1543, the switch control block also turns off
Q2 and turns on Q3 when the input voltage falls below
the UVLO threshold (Figure 4).
Oscillator Frequency (FREQ)
The MAX1542 internal oscillator is preset to 1.2MHz. The
internal oscillator frequency is pin programmable for the
MAX1543. Connect FREQ to ground or leave it uncon-
nected for 640kHz operation and connect it to V
IN
for
1.2MHz operation. FREQ has a 5µA (typ) pulldown current.
Fault Protection
Once the soft-start routine is complete, if the output of
the main regulator is below the fault detection threshold,
the MAX1542/MAX1543 activate the fault timer. If the
fault condition continuously exists throughout the fault
timer duration, the MAX1542/MAX1543 set the fault
latch, which shuts down the device. After removing the
fault condition, cycle the input voltage (IN) below V
UVLO
to clear the fault latch and reactivate the device.
Digital Soft-Start
The MAX1542/MAX1543 digital soft-start period dura-
tion is 14ms (typ). During this time, the MAX1542/
MAX1543 directly limit the peak inductor current, allow-
ing from zero up to the full current-limit value in eight
equal current steps (I
LIM
/8). The maximum load current
is available after output voltage reaches the full regula-
tion threshold (which terminates soft-start), or after the
soft-start timer expires.
LX
IN
AGND
PGND
DEL
CTL
DRN
SRC
COMP
POS2
OUT2
POS1
OUT1
NEG1
SUP
FB
R8
100k
Ω
C10
33nF
D1
V
IN
2.6V TO 5.5V
V
MAIN
+8V AT 250mA
TO VCOM
BACKPLANE
D2
D3
D4
C6
0.1
µF
C8
4.7
µF
C9
4.7
µF
C7
0.1
µF
C11
220pF
C1
10
µF
C2
0.1
µF
C3
0.1
µF
C5
0.1
µF
C4
0.1
µF
R1
75k
Ω
R2
13.7k
Ω
R3
40k
Ω
R5
40k
Ω
R6
40k
Ω
R4
40k
Ω
-7V AT 20mA
G_OFF
NEG2
G_ON
+22V AT 20mA
FREQ
COM
L1
4.7
µF
MAX1543
Figure 2. MAX1543 Typical Application Circuit