LM2648 National Semiconductor, LM2648 Datasheet - Page 16

• Manufacturer Part Number: LM2648
• Description: Synchronous Step-Down 3-Channel Switching Regulator Controller
• Manufacturer: National Semiconductor

www.national.com
Operation Descriptions
ACTIVE OUTPUT DISCHARGE
Each channel has an embedded MOSFET with the drain
connected to the SWx pin to provide a smooth controlled
shutdown ramp. Each MOSFET will discharge the output
capacitor of its channel if the device enters a fault state
caused by one of the following conditions:
1. UVP or Prot-in2
2. UVLO (Internal or External)
3. Thermal shut-down (TSD)
4. Prot-in1
The MOSFETS provide 500Ω of discharge resistivity for
channels 1 and 2 and 200Ω for channel 3.
Channel 3 has a secondary quick discharge feature. When
the channel 3 output voltage falls below 65% of nominal,
LDRV3a is turned on (LDRV3b remains off). This provides a
faster discharge for the second half of the channel 3 shut-
down process.
CURRENT SENSING
As shown in Figure 2, the KSx and RSNSx pins are the
inputs of the current sense amplifier for channel 3. Current
sensing is accomplished by sensing the voltage across a
current sense resistor connected from V
top FET. Keeping the differential current-sense voltage be-
low 200mV ensures linear operation of the current sense
amplifier. Therefore, the current sense resistor must be small
enough so that the current sense voltage does not exceed
200mV when the top FET is on. There is a leading edge
blanking circuit that forces the top FET on for at least 166ns.
Beyond this minimum on time, the output of the PWM com-
parator is used to turn off the top FET. Additionally, a mini-
mum voltage of at least 50mV across the sense resistor is
recommended to ensure a high SNR at the current sense
amplifier.
Assuming a maximum sense voltage of 200mV, the current
sense resistor can be calculated as follows:
where Imax is the maximum expected load current, including
overload multiplier (ie:120%), and Irip is the inductor ripple
current (See Inductor Selection below). The above equation
gives the maximum recommended value for the sense resis-
tor, Rsns. Conduction losses will increase with Rsns, thus
lowering efficiency.
To ensure accurate current sensing, special attention in
board layout is required. The KSx and RSNSx pins require
separate traces to form a Kelvin connection to the corre-
sponding current sense nodes.
Current sensing on channels 1 and 2 is used only to monitor
the current limit. Current is sensed identical to channel 3,
except that it does not require a KS pin and there is no
blanking time. As with channel 3, RSNSx is connected to the
bottom side of the current sense resistor. The top side of the
current sense resistor connects to both VIN and the current
limit resistor. Both KSx and RSNSx must be connected with
separate traces to ensure accurate current limiting.
IN
(Continued)
to the drain of the
16
CURRENT LIMITING
The peak current limit is set with an external resistor con-
nected between the ILIMx pin and the top of the current
sense resistor. An adjustable internal current sink on the
ILIMx pin produces a voltage across the ILIM resistor to set
the current limit threshold. This voltage is compared with the
current sense voltage. A 10nF capacitor across the resistor
is required to filter unwanted noise that could improperly trip
the current limit comparator.
Current limit is activated when the inductor current is high
enough to cause the voltage across the current sense resis-
tor to exceed the voltage across the current limit resistor.
This will toggle the comparator, which turns off the top FET
immediately. The current limit comparator is disabled when
the top FET is turned off and during the leading edge blank-
ing time. The equation for current limit resistor, Rlim, is as
follows:
Where I
comparator will be tripped, and I
current (20µA typical). This calculated Rlim value guarantees
that the minimum current limit will not be less than I
recommended that a 1% tolerance resistor be used.
REFERENCE CURRENT SETTING
The ILIMx current sink value can be adjusted using the
RILIM pin. The RILIM pin is connected through a resistor to
SGND to set the reference current at the current limit pins
(ILIMx). The resistor value can be determined using the
following equation:
Where I
default value of 61.9kΩ is used, which provides the recom-
mended reference current of 20µA. Once an initial value is
selected, R
limit setting on all three channels.
FIGURE 2. Ch.3 Current Sense and Current Limit
max
REF
IREF
is the load current at which the current limit
is the desired reference current. Generally, a
can be adjusted to precisely trim the current
REF
is the reference sink
20059610
max
. It is