參數(shù)資料
型號： TPS51511RHLT
廠商： TEXAS INSTRUMENTS INC
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 350 kHz SWITCHING FREQ-MAX, PQCC20
封裝： ROHS COMPLIANT, PLASTIC, QFN-20
文件頁數(shù)： 5/29頁
文件大?。?/td> 1019K
代理商： TPS51511RHLT
www.ti.com
VOSW Switcher, Light Load Condition
I
OUT(LL) +
1
2
L
V
IN * VOUT
V
OUT
V
IN
(1)
Low-Side Driver
High-Side Driver
Current Sensing Scheme
SLVS735A – FEBRUARY 2007 – REVISED APRIL 2007
TPS51511 automatically reduces the switching frequency under light load conditions to maintain high efficiency.
This frequency reduction is achieved smoothly and without increasing the VOUT ripple or affecting the load
regulation.
As the output current decreases from a heavy load condition, the inductor current is also reduced and eventually
comes to the point that its ripple-valley value decreases down to zero current, which is the boundary between
continuous conduction and discontinuous conduction modes. The rectifying MOSFET will be turned off when this
zero
inductor
current
is
detected.
As
the
load
current
decreases
further,
the
converter
runs
in
discontinuous-conduction mode, and takes longer to discharge the output capacitor to the level that will issue
the next ON cycle. The ON time is kept the same as that in the heavy load condition. Conversely, when the
output current increases from a light load to a heavy load, the switching frequency increases to the constant 350
kHz as the inductor current reaches continuous conduction. The transition point between light and heavy-load
operation IOUT(LL) (i.e., the threshold between continuous and discontinuous conduction mode) can be calculated
as follows;
Where f is the PWM switching frequency, 350 kHz.
The switching frequency versus the output current in the light load condition is a function of L, f, VIN and VOUT,
but it decreases almost proportionally to the output current from the IOUT(LL) given above. For example, it will be
35 kHz at IOUT(LL)/10 and 3.5 kHz at IOUT(LL)/100.
The low-side driver is designed to drive high current low RDS(ON) N-channel MOSFETs. The drive capability is
represented by its internal resistance, 3
for V5IN to DRVL, and 0.9 for DRVL to PGND. A dead time to
prevent shoot-through is internally generated between the top MOSFET off to bottom MOSFET on, and bottom
MOSFET off to top MOSFET on. The 5-V bias voltage is delivered from V5IN supply.
The instantaneous drive current is supplied by an input capacitor connected between V5IN and GND. Add a
ceramic capacitor with a value between 1.0
F and 4.7 F placed close to the V5IN pin to stabilize the 5-V
output from any parasitic impedance from the supply. The average drive current is equal to the gate charge at
VGS=5 V condition, times the switching frequency. This gate-drive current as well as the high-side gate-drive
current times 5 V accounts for the power that must be dissipated from the TPS51511 package.
The high-side driver is designed to drive high-current, low RDS(ON) N-channel MOSFET(s). It is configured as a
floating driver, with the 5-V bias voltage delivered from the V5IN supply. The average drive current is also
calculated by the gate charge at VGS=5 V condition, times switching frequency. The instantaneous drive current
is supplied by the flying capacitor between VBST and LL pins. The drive capability is represented by its internal
resistance, 3
for VBST to DRVH, and 0.9 for DRVH to LL.
To support both high-accuracy and low-cost current sensing applications, TPS51511 supports two different
current sensing schemes; external-resistor sensing, and rectifying-MOSFET RDS(ON) sensing.
In the resistor-sensing scheme, an appropriate current-sensing resistor is connected between the source
terminal of the rectifying MOSFET and PGND, and the CS pin is connected to the rectifying MOSFET source
terminal node. The inductor current is monitored by the voltage between PGND pin and CS pin.
In the RDS(ON)-sensing scheme, the CS pin is connected to the V5IN pin through the trip-voltage setting resistor
RTRIP. In this case, the CS terminal sinks the 10-A ITRIP current, and the trip level is set to the voltage across
RTRIP. The inductor current is monitored by the voltage between the PGND pin and the LL pin, so the LL pin is
connected to the drain terminal of the rectifying MOSFET. ITRIP has a 4500-ppm/°C temperature slope to
compensate the temperature dependency of the RDS(ON).
In either scheme, PGND is used as the positive current-sensing node, so PGND must be connected to the
proper current-sensing device, i.e., the sense resistor or the source terminal of the rectifying MOSFET.
13
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