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L6919C
8/32
DEVICE DESCRIPTION
The device is an integrated circuit realized in BCD technology. It provides complete control logic and protections
for a high performance dual-phase step-down DC-DC converter optimized for microprocessor power supply. It
is designed to drive N Channel MOSFETs in a dual-phase synchronous-rectified buck topology. A 180 deg
phase shift is provided between the two phases allowing reduction in the input capacitor current ripple, reducing
also the size and the losses. The output voltage of the converter can be precisely regulated, programming the
VID pins, from 0.800V to 1.550V with 25mV binary steps, with a maximum tolerance of ±0.6% over temperature
and line voltage variations. The device manages On-The-Fly VID Code changes stepping to the new configu-
ration following the VID table with no need for external components. The device provides an average current-
mode control with fast transient response. It includes a 200kHz free-running oscillator. The error amplifier fea-
tures a 15V/
μ
s slew rate that permits high converter bandwidth for fast transient performances. Current infor-
mation is read across the lower mosfets R
dsON
or across a sense resistor in fully differential mode. The current
information corrects the PWM output in order to equalize the average current carried by each phase. Current
sharing between the two phases is then limited at ±10% over static and dynamic conditions. The device protects
against Over-Current, with an OC threshold for each phase, entering in constant current mode. Since the current
is read across the low side mosfets, the constant current keeps constant the bottom of the inductors current
triangular waveform. When an under voltage is detected the device latches and the FAULT pin is driven high.
The device performs also Over-Voltage protection that disables immediately the device turning ON the lower
driver and driving high the FAULT pin.
OSCILLATOR
The switching frequency is internally fixed to 200kHz. The internal oscillator generates the triangular waveform for the
PWM charging and discharging with a constant current an internal capacitor. The current delivered to the oscillator is
typically 17
μ
A (Fsw=200KHz) and may be varied using an external resistor (ROSC) connected between OSC pin and
GND or Vcc. Since the OSC pin is maintained at fixed voltage (Typ. 1.235V), the frequency is varied proportionally to
the current sunk (forced) from (into) the pin considering the internal gain of 12KHz/
μ
A.
In particular connecting it to GND the frequency is increased (current is sunk from the pin), while connecting ROSC
to Vcc=12V the frequency is reduced (current is forced into the pin), according to the following relationships:
Note that forcing a 17
μ
A current into this pin, the device stops switching because no current is delivered to the
oscillator.
Figure 1. R
OSC
vs. Switching Frequency
R
OSC
vs. GND: f
S
200kHz
R
OSC
K
(
)
-----------------------------
12kHz
A
----------
+
200kHz
6
R
OSC
K
)
-----------------------------
+
=
=
R
OSC
vs. 12V: f
S
200kHz
12
R
OSC
K
(
)
--------–
12kHz
A
----------
–
200kHz
7
R
OSC
K
(
)
-------------------------------
–
=
=
0
1000
2000
3000
4000
5000
6000
7000
0
50
100
150
200
Frequency (KHz)
R
)
0
200
400
600
800
1000
200
300
400
500
600
Frequency (KHz)
R
)