Datasheet

switched at 500kHz, large MOSFETs with a total of 40nC
total gate charge would require 40nC × 500kHz, which is
approximately 20mA.
On/Off Control (EN)
The EN pin has an accurate 1.238V (typ) rising threshold
with 5% hysteresis. The accurate threshold allows it to
be used to monitor the input voltage or other analog sig-
nals of interest. If V
EN
voltage is less than its threshold,
then the step-down regulator and all linear regulators are
turned off. VL and the internal reference remain active
when EN is low to allow an accurate EN threshold.A rising
edge on the pin clears any latched faults except for a ther-
mal fault, which is cleared only by cycling the input power.
Undervoltage Lockout
If VL drops below 3.4V (typ), the MAX1530/MAX1531
assume that the supply voltage is too low to make valid
decisions. Therefore, the undervoltage lockout (UVLO)
circuitry turns off all the internal bias supplies. Switching
is inhibited, and the DL and DH gate drivers are forced
low. After VL rises above 3.5V (typ), the fault and thermal
shutdown latches are cleared and startup begins if EN is
above its threshold.
Startup Sequence (ONL_, SEQ)
The MAX1530/MAX1531 are not enabled unless all four
of the following conditions are met: 1) VL exceeds the
UVLO threshold, 2) EN is above 1.238V, 3) the fault latch
is not set, and 4) the thermal shutdown latch is not set.
After all four conditions are met, the step-down controller
starts switching and enables soft-start (Figure 5). After
the step-down regulator soft-start is done, the lowvoltage
logic linear regulator controller (LR1) soft-starts.
The remaining linear regulator controllers and the
sequence block that can be used to control them are
enabled at the same time as the step-down regulator. The
SEQ logic input is used in combination with the ONL_ pins
to control the startup sequence. When SEQ is high and
the sequence block is enabled, each ONL_ pin sources
2μA (typ). When the voltage on an ONL_ pin reaches
1.238V (typ), its respective linear regulator controller
(LR_) is enabled. When SEQ is low or the sequence block
is not enabled, each ONL_ pin is connected to ground
through a 1.5kΩ internal MOSFET.
The sequence block allows the user to program the start-
up of LR2 to LR5 in any desired sequence. If no capacitor
is placed on an ONL_ pin, its LR_ controller starts imme-
diately after the sequence block is enabled and SEQ goes
high. Placing a 1.5nF capacitor on an ONL_ pin provides
about 1ms delay for the respective LR_ controller. Placing
different size capacitors on each ONL_ pin allows any
arbitrary startup sequence.
An arbitrary startup sequence can also be created witha
single capacitor (Figure 6). Capacitor C1, together with
the 8μA current (2μA per ONL_ pin), is chosen to provide
Figure 5. Startup Conditions
Figure 6. Single-Capacitor Sequence Configuration
ONL_
CURRENT
SOURCES ON
SEQUENCE
BLOCK
ENABLED
LR5
STARTUP
LR4
STARTUP
LR2
STARTUP
LR1
STARTUP
SEQ = HIGH
ONL5 > 1.24V
ONL2 > 1.24V ONL4 > 1.24V
LR3
STARTUP
STEP-DOWN
REGULATOR
STARTUP
STEP-DOWN
SOFT-START
DONE
EN > 1.24V
AND
VL > 3.5V
ONL3 > 1.24V
SEQ
ONL_
0V
1.238V
5V
LRa
LRb
LRc
LRd
ON
ON
ON
ON
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
16ms
ONLa ONLb ONLc ONLd
ONLa ONLb ONLc ONLd
R2
75kΩ
R3
150kΩ
R1
51kΩ
C1
0.1µF
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
www.maximintegrated.com
Maxim Integrated │ 21