Datasheet

LTC3736-2
12
37362fb
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 external P-channel MOSFET is
determined by:
Δ =
()
V
AV V
SENSE MAX
ITH
()
–.07
10
where A is a constant determined by the state of the IPRG
pins. Floating the IPRG pin selects A = 1.875; tying IPRG
to V
IN
selects A = 2.7; tying IPRG to SGND selects A = 1.3.
The maximum value of V
ITH
is typically about 1.98V, so
the maximum sense voltage allowed across the external
P-channel MOSFET is 240mV, 345mV, or 167mV 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 external P-channel
MOSFET:
I
V
R
PK
SENSE MAX
DS ON
=
Δ
()
()
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 LTC3736-2
is shut down or in undervoltage lockout.
2-Phase Operation
Why the need for 2-phase operation? Until recently,
constant-frequency dual switching regulators operated
both controllers in phase (i.e., single-phase operation).
This means that both topside MOSFETs (P-channel) 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 fl owing 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 LTC3736-2
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 signifi cant 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 effi ciency.
OPERATION
Figure 1. Maximum Peak Current vs Duty Cycle
DUTY CYCLE (%)
10
SF = I/I
MAX
(%)
60
80
110
100
90
37362 F01
40
20
50
70
90
30
10
0
30
50
70
200
40
60
80
100
(Refer to Functional Diagram)