Rainbow Electronics MAX786 - Manual (Page 10)

MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
10 ______________________________________________________________________________________
+5V Switch-Mode Supply
The +5V output is produced by a current-mode, PWM
step-down regulator, which is nearly identical to the
+3.3V supply. The +5V supply’s dropout voltage, as
configured in Figure 1, is typically 400mV at 2A. As V+
approaches 5V, the +5V output gracefully falls with
V+ until the VL regulator output hits its undervoltage-
lockout threshold at 4V. At this point, the +5V supply
turns off.
The default frequency for both PWM controllers is
300kHz (with SYNC connected to REF), but 200kHz
may be used by connecting SYNC to GND or VL.
+3.3V and +5V PWM Buck Controllers
The two current-mode PWM controllers are identical
except for different preset output voltages (Figure 3).
Each PWM is independent except for being synchro-
nized to a master oscillator and sharing a common ref-
erence (REF) and logic supply (VL). Each PWM can
be turned on and off separately via ON3 and ON5. The
PWMs are a direct-summing type, lacking a tradi-
tional integrator error amplifier and the phase shift
associated with it. They therefore do not require any
external feedback compensation components if the fil-
ter capacitor ESR guidelines given in the
Design
Procedure are followed.
The main gain block is an open-loop comparator that
sums four input signals: an output voltage error signal,
current-sense signal, slope-compensation ramp, and
precision voltage reference. This direct-summing
method approaches the ideal of cycle-by-cycle control
of the output voltage. Under heavy loads, the controller
operates in full PWM mode. Every pulse from the oscil-
lator sets the output latch and turns on the high-side
switch for a period determined by the duty cycle
(approximately V
OUT
/V
IN
). As the high-side switch turns
off, the synchronous rectifier latch is set and, 60ns later,
the low-side switch turns on (and stays on until the
beginning of the next clock cycle, in continuous mode,
or until the inductor current crosses through zero, in
discontinuous mode). Under fault conditions where the
inductor current exceeds the 100mV current-limit
threshold, the high-side latch is reset and the high-side
switch is turned off.
At light loads, the inductor current fails to exceed the
25mV threshold set by the minimum current comparator.
When this occurs, the PWM goes into idle mode, skip-
ping most of the oscillator pulses in order to reduce the
switching frequency and cut back switching losses. The
oscillator is effectively gated off at light loads because
the minimum current comparator immediately resets the
high-side latch at the beginning of each cycle, unless the
FB_ signal falls below the reference voltage level.
Soft-Start/SS_ Inputs
Connecting capacitors to SS3 and SS5 allows gradual
build-up of the +3.3V and +5V supplies after ON3 and
ON5 are driven high. When ON3 or ON5 is low, the
appropriate SS capacitors are discharged to GND.
When ON3 or ON5 is driven high, a 4µA constant cur-
rent source charges these capacitors up to 4V. The
resulting ramp voltage on the SS_ pins linearly increas-
es the current-limit comparator setpoint so as to
increase the duty cycle to the external power MOSFETs
up to the maximum output. With no SS capacitors, the
circuit will reach maximum current limit within 10µs.
Soft-start greatly reduces initial in-rush current peaks
and allows start-up time to be programmed externally.
Synchronous Rectifiers
Synchronous rectification allows for high efficiency
by reducing the losses associated with the Schottky
rectifiers.
When the external power MOSFET N1 (or N2) turns off,
energy stored in the inductor causes its terminal volt-
age to reverse instantly. Current flows in the loop
formed by the inductor, Schottky diode, and load — an
action that charges up the filter capacitor. The Schottky
diode has a forward voltage of about 0.5V which,
although small, represents a significant power loss,
degrading efficiency. A synchronous rectifier, N3 (or
N4), parallels the diode and is turned on by DL3 (or
DL5) shortly after the diode conducts. Since the on
resistance (r
DS(ON)
) of the synchronous rectifier is very
low, the losses are reduced.
The synchronous rectifier MOSFET is turned off when
the inductor current falls to zero.
Cross conduction (or “shoot-through”) occurs if the
high-side switch turns on at the same time as the syn-
chronous rectifier. The MAX786’s internal break-before-
make timing ensures that shoot-through does not occur.
The Schottky rectifier conducts during the time that nei-
ther MOSFET is on, which improves efficiency by pre-
venting the synchronous-rectifier MOSFET’s lossy body
diode from conducting.
The synchronous rectifier works under all operating condi-
tions, including discontinuous-conduction and idle mode.
Boost Gate-Driver Supply
Gate-drive voltage for the high-side N-channel switch is
generated with a flying-capacitor boost circuit as shown
in Figure 4. The capacitor is alternately charged from
the VL supply via the diode and placed in parallel with
the high-side MOSFET’s gate-source terminals. On start-
up, the synchronous rectifier (low-side) MOSFET forces
LX_ to 0V and charges the BST_ capacitor to 5V. On the