ML4828 Ver la hoja de datos (PDF) - Micro Linear Corporation
Número de pieza
componentes Descripción
Fabricante
ML4828 Datasheet PDF : 11 Pages
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ML4828
If the current limit condition persists, then IRST will be
reactivated, thus charging CRST to a higher level as shown
in Figure 7. Eventually, the voltage at RST will exceed
2.5V, and the soft start comparator will trip, shutting down
all power drivers and inhibiting any further delivery of
power. At the same time, the soft start capacitor CSS is
discharged to prepare for the next start up cycle.
1V
V(PIN 20)
2.5V
V(PIN 12)
Figure 7. ILIM and Resulting RCRST
Waveforms During Short Circuit.
During the ILIM shutdown, IRST is turned off, and CRST is
discharged through RRST with a time constant of:
tRST = RRST × CRST
(12)
When the condition causing the current limit is removed,
RRST will discharge CRST with a time constant of tRST.
When the voltage at RST (pin 12) drops to 1.25V, the soft
start comparator and the converter will undergo a start up
cycle. The restart delay (tD(RST)) is given by:
tD(RST) = tRST × 1.39
(13)
For example, with CRST = 25µF and RRST = 240kΩ:
dv = 500µA = 20 V
dt 25µF
s
(14)
and
tD(RST) = (240kΩ × 25µF) × 1.39 = 8.3s
(15)
Since the threshold for shutdown is 2.5V, the controller
will shut down after approximately 125ms. After the
converter recovers form the current limit condition, the
controller will reactivate after 8.3s.
UNDERVOLTAGE LOCKOUT
During start-up, the ML4828 draws very little current
(typically 150µA) and VREF is disabled. When VCC rises
above 6.0V, the internal circuitry and VREF are enabled,
and will stay enabled until VCC falls below the 4.5V UV
lockout threshold.
SHUTDOWN FUNCTION
The ML4828 can be externally shut down by bringing
SDN (pin 19) low. The shutdown threshold (VSD) is given
by
VSD = 0.33 × VCC
(16)
For example, if VCC= 5V, then VSD = 1.67V. As long as
2.4V < VCC < 6.0V, the SDN pin will be TTL compatible.
8
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