LTC3836_15 Datasheet, PDF (12/30 Pages) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

LTC3836
OPERATION (Refer to Functional Diagram)
Undervoltage Lockout
To prevent operation of the external MOSFETs below safe
input voltage levels, an undervoltage lockout is incorpo-
rated in the LTC3836. When the input supply voltage (V_IN)
drops below 2.25V, the external MOSFETs and all internal
circuitry are turned off except for the undervoltage block,
which draws only a few microamperes.
Peak Current Sense Voltage Selection and Slope
Compensation (IPRG1 and IPRG2 Pins)
When a controller is operating below 20% duty cycle,
the peak current sense voltage (between the SENSE⁺ and
SW pins) allowed across the main N-channel MOSFET is
determined by:
( ) A

V_SENSE(MAX) =
V_ITH – 0.7V
10
where A is a constant determined by the state of the
IPRG pins. Floating the IPRG pin selects A = 1;
tying IPRG to V_IN selects A = 5/3; tying IPRG to SGND
selects A = 2/3. The maximum value of V_ITH is typically
about 1.98V, so the maximum sense voltage allowed across
the main N-channel MOSFET is 122mV, 202mV, or 82mV
for the three respective states of the IPRG pin. The peak
sense voltages for the two controllers can be independently
selected by the IPRG1 and IPRG2 pins.
However, once the controller’s duty cycle exceeds 20%,
slope compensation begins and effectively reduces the
peak sense voltage by a scale factor given by the curve
in Figure 1.
The peak inductor current is determined by the peak
sense voltage and the on-resistance of the main N-chan-
nel MOSFET:
I_PK
=

V_SENSE(MAX)
R_DS(ON)
12
110
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
37362 F01
Figure 1. Maximum Peak Current vs Duty Cycle
Power-Good (PGOOD) Pin
A window comparator monitors both feedback voltages
and the open-drain PGOOD output pin is pulled low when
either or both feedback voltages are not within ±10% of the
0.6V reference voltage. PGOOD is low when the LTC3836
is shut down or in undervoltage lockout.
2-Phase Operation
Why the need for 2-phase operation? Many constant-fre-
quency dual switching regulators operate both controllers
in phase (i.e., single phase operation). This means that
both topside MOSFETs are turned on at the same time,
causing current pulses of up to twice the amplitude of
those from a single regulator to be drawn from the input
capacitor. These large amplitude pulses increase the total
RMS current flowing in the input capacitor, requiring the
use of larger and more expensive input capacitors, and
increase both EMI and power losses in the input capacitor
and input power supply.
With 2-phase operation, the two controllers of the LTC3836
are operated 180 degrees out-of-phase. This effectively
interleaves the current pulses coming from the topside
MOSFET switches, greatly reducing the time where they
overlap and add together. The result is a significant reduc-
tion in the total RMS current, which in turn allows the
use of smaller, less expensive input capacitors, reduces
shielding requirements for EMI and improves real world
operating efficiency.
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