LTC3562 Datasheet, PDF (10/20 Pages) Linear Technology – I2C Quad Synchronous Step-Down DC/DC Regulator 2 × 600mA, 2 × 400mA

LTC3562
OPERATION
V_OUT Adjustable (Type-B) Regulators
Unlike the Type-A regulators, the two Type-B regulators
do not require an external resistor divider network to
program its output voltage. Regulators R600B and R400B
have feedback resistor networks internal to the chip whose
values can be adjusted through I²C control. These inter-
nal feedback resistors can be configured such that the
output voltages can be programmed directly. The output
voltages can be programmed from 600mV to 3.775V in
25mV increments.
Pins OUT600B and OUT400B are feedback sense pins that
connect to the top of the internal resistor divider networks.
These output pins should sense the output voltages of
the regulators right at the output capacitor C_O (after the
inductor), as illustrated in Figure 2.
The maximum operating current for regulators R600B and
R400B are 600mA and 400mA, respectively. The Type-B
regulators do not have individual run pins as do the Type-A
regulators. Thus regulators R600B and R400B can only
be enabled through control of the I²C port. When the
part initially powers up, the Type-B regulators default to
shutdown mode and remain disabled until programmed
through I²C.
Regulator Operating Modes
All of the LTC3562’s switching regulators include four
possible operating modes to meet the noise/power needs
of a variety of applications.
In pulse skip mode, an internal latch is set at the start of
every cycle which turns on the main P-channel MOSFET
switch. During each cycle, a current comparator compares
the peak inductor current to the output of an error amplifier.
The output of the current comparator resets the internal
latch which causes the main P-channel MOSFET switch to
turn off and the N-channel MOSFET synchronous rectifier
to turn on. The N-channel MOSFET synchronous rectifier
turns off at the end of the 2.25MHz cycle or if the current
through the N-channel MOSFET synchronous rectifier
drops to zero. Using this method of operation, the error
amplifier adjusts the peak inductor current to deliver the
required output power. All necessary compensation is
internal to the switching regulator requiring only a single
ceramic output capacitor for stability. At light loads in
pulse skip mode, the inductor current may reach zero
on each pulse which will turn off the N-channel MOSFET
synchronous rectifier. In this case, the switch node (SW)
goes high impedance and the switch node voltage will
“ring.” This is discontinuous mode operation, and is
normal behavior for a switching regulator. At very light
loads in pulse skip mode, the switching regulators will
automatically skip pulses as needed to maintain output
regulation. At high duty cycle (V_OUT > V_IN/2) it is possible
for the inductor current to reverse at light loads, causing
the step-down switching regulator to operate continuously.
When operating continuously, regulation and low noise
output voltage are maintained, but input operating current
will increase to a couple mA.
In forced Burst Mode operation, the switching regulators
use a constant-current algorithm to control the inductor
current. By controlling the inductor current directly and
using a hysteretic control loop, both noise and switch-
ing losses are minimized. In this mode output power is
limited. While operating in forced Burst Mode operation,
10
LTC3562
SWxB
OUTxB
GND
L
600mV to 3.775V
C_O
3562 F02
Figure 2. Type-B Regular Application Circuit
3562fa