RT9218B Ver la hoja de datos (PDF) - Richtek Technology
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RT9218B Datasheet PDF : 17 Pages
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RT9218B
Under Voltage Protection
The voltage at FB and FBL pin is monitored and protected
against UV (under voltage). The UV threshold is the FB or
FBL under 75%. UV detection has 30μs triggered delay.
When OC or UV_FBL is trigged, a hiccup restart sequence
will be initialized, as shown in Figure 7 Only 4 times of
trigger are allowed to latch off. Hiccup is disabled during
soft-start interval, but UV_FB has some difference from
OC and UV_FBL, it will always trigger VIN power sensing
after 4 times hiccup, as shown in Figure 8.
COUNT = 1
4V
2V
0V
COUNT = 2 COUNT = 3
OVERLOAD
APPLIED
COUNT = 4
0A
T0 T1
T2
T3
T4
TIME
Figure 7. UV and OC trigger hiccup mode
Power Off
UGATE
FB
VOUT
VIN
(20V/Div)
(500mV/Div)
UV
VIN Power
Sensing
(2V/Div)
IOUT = 2A
(2V/Div)
Time (10ms/Div)
Figure 8, UV_FB trigger VIN power sensing
LDO Power Sequence
In VGA field, the MOSFET of LVOUT is sourced by external
voltage not by SVOUT.
This connection may trigger UV protection to shutdown
RT9218B, but using the typical application circuit won't
have this issue. See figure 9 using OPS pin to control the
power sequence.
VIN_SW (5V/12V)
VIN_LDO (3.3V)
Shutdown
Enable
OPS_Disable
Figure 9. LDO power sequence
PWM Layout Considerations
MOSFETs switch very fast and efficiently. The speed with
which the current transitions from one device to another
causes voltage spikes across the interconnecting
impedances and parasitic circuit elements. The voltage
spikes can degrade efficiency and radiate noise, that results
in over-voltage stress on devices. Careful component
placement layout and printed circuit design can minimize
the voltage spikes induced in the converter. Consider, as
an example, the turn-off transition of the upper MOSFET
prior to turn-off, the upper MOSFET was carrying the full
load current. During turn-off, current stops flowing in the
upper MOSFET and is picked up by the low side MOSFET
or schottky diode. Any inductance in the switched current
path generates a large voltage spike during the switching
interval. Careful component selections, layout of the critical
components, and use shorter and wider PCB traces help
in minimizing the magnitude of voltage spikes.
There are two sets of critical components in a DC-DC
converter using the RT9218B. The switching power
components are most critical because they switch large
amounts of energy, and as such, they tend to generate
equally large amounts of noise. The critical small signal
components are those connected to sensitive nodes or
those supplying critical bypass current.
The power components and the PWM controller should
be placed firstly. Place the input capacitors, especially the
high-frequency ceramic decoupling capacitors, close to the
power switches. Place the output inductor and output
capacitors between the MOSFETs and the load. Also locate
the PWM controller near by MOSFETs.
A multi-layer printed circuit board is recommended.
DS9218B-09 March 2007
www.richtek.com
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