lm25119psqx National Semiconductor Corporation, lm25119psqx Datasheet - Page 12

• Manufacturer Part Number: lm25119psqx
• Description: Lm25119 Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Enable 2
The LM25119 contains an enable function allowing shutdown
control of channel2, independent of channel1. If the EN2 pin
is pulled below 2.0V, channel2 enters shutdown mode. If the
EN2 input is greater than 2.5V, channel2 returns to normal
operation. An internal 50kΩ pull-up resistor on the EN2 pin
allows this pin to be left floating for normal operation. The EN2
input can be used in conjunction with the UVLO pin to se-
quence the two regulator channels. If EN2 is held low as the
UVLO pin increases to a voltage greater than the 1.25V UVLO
threshold, channel1 will begin operation while channel2 re-
mains off. Both channels become operational when the UV-
LO, EN2, VCC1, and VCC2 pins are above their respective
operating thresholds. Either channel of the LM25119 can also
be disabled independently by pulling the corresponding SS
pin to AGND.
Oscillator and Sync Capability
The LM25119 switching frequency is set by a single external
resistor connected between the RT pin and the AGND pin
(R
and connected directly to the pins of the IC (RT and AGND).
To set a desired switching frequency (f
the resistor can be calculated from the following equation:
Where RT is in ohms and f
is the output switching frequency of each channel. The inter-
nal oscillator runs at twice the switching frequency and an
internal frequency divider interleaves the two channels with
180° phase shift between PWM pulses at the HO pins.
The RT pin can be used to synchronize the internal oscillator
to an external clock. The internal oscillator can be synchro-
nized by AC coupling a positive edge into the RT pin. The
voltage at the RT pin is nominally 1.25V and the voltage at
the RT pin must exceed 4V to trip the internal synchronization
pulse detector. A 5V amplitude signal and 100pF coupling
capacitor are recommended. Synchronizing at greater than
twice the free-running frequency may result in abnormal be-
havior of the pulse width modulator. Also, note that the output
switching frequency of each channel will be one-half the ap-
plied synchronization frequency.
Error Amplifiers and PWM
Comparators
Each of the two internal high-gain error amplifiers generates
an error signal proportional to the difference between the reg-
ulated output voltage and an internal precision reference
(0.8V). The output of each error amplifier is connected to the
COMP pin allowing the user to provide loop compensation
components. Generally a Type II network is recommended.
This network creates a pole at 0Hz, a mid-band zero, and a
T
). The resistor should be located very close to the device
SW
is in Hertz. The frequency f
SW
) of each channel,
(1)
SW
12
noise reducing high frequency pole. The PWM comparator
compares the emulated current sense signal from the RAMP
generator to the error amplifier output voltage at the COMP
pin. Only one error amplifier is required when configuring the
controller as a two channel, single output interleaved regula-
tor. For these applications, the channel1 error amplifier (FB1,
COMP1) is configured as the master error amplifier. The
channel2 error amplifier must be disabled by connecting the
FB2 pin to the VCC2 pin. When configured in this manner the
output of the channel2 error amplifier (COMP2) will be dis-
abled and have a high output impedance. To complete the
interleaved configuration the COMP1 and the COMP2 pins
should be connected together to facilitate PWM control of
channel2 and current sharing between channels.
Ramp Generator
The ramp signal used in the pulse width modulator for current
mode control is typically derived directly from the buck switch
current. This switch current corresponds to the positive slope
portion of the inductor current. Using this signal for the PWM
ramp simplifies the control loop transfer function to a single
pole response and provides inherent input voltage feed-for-
ward compensation. The disadvantage of using the buck
switch current signal for PWM control is the large leading
edge spike due to circuit parasitics that must be filtered or
blanked. Also, the current measurement may introduce sig-
nificant propagation delays. The filtering, blanking time and
propagation delay limit the minimum achievable pulse width.
In applications where the input voltage may be relatively large
in comparison to the output voltage, controlling small pulse
widths and duty cycles are necessary for regulation. The
LM25119 utilizes a unique ramp generator which does not
actually measure the buck switch current but rather recon-
structs the signal. Representing or emulating the inductor
current provides a ramp signal to the PWM comparator that
is free of leading edge spikes and measurement or filtering
delays. The current reconstruction is comprised of two ele-
ments; a sample-and-hold DC level and the emulated induc-
tor current ramp as shown in
FIGURE 3. Composition of Current Sense Signal
Figure
3.
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