Rainbow Electronics MAX776 - Manual (Page 10)

MAX774/MAX775/MAX776
With light loads, the MOSFET switches on for one or
more cycles and then switches off, much like in tradi-
tional PFM converters. To increase light-load efficiency,
the current limit for the first two pulses is set to one-half
the peak current limit. If those pulses bring the output
voltage into regulation, the voltage comparator keeps
the MOSFET off, and the current limit remains at one-half
the peak current limit. If the output voltage is out of
regulation after two consecutive pulses, the current limit
for the next pulse will equal the full current limit.
With heavy loads, the MOSFET first switches twice at
one-half the peak current value. Subsequently, it stays
on until the switch current reaches the full current limit,
and then turns off. After it is off for 2.3µs, the MOSFET
switches on once more, and remains on until the switch
current again reaches its limit. This cycle repeats until
the output is in regulation.
A benefit of this control scheme is that it is highly effi-
cient over a wide range of input/output ratios and load
currents. Additionally, PFM converters do not operate
with constant-frequency switching, and have relaxed
stability criterion (unlike PWM converters). As a result,
their external components require smaller values.
With PFM converters, the output voltage ripple is not
concentrated at the oscillator frequency (as it is with
PWM converters). So for applications where the ripple
frequency is important, the PWM control scheme must
be used. However, for many other applications, the
smaller capacitors and lower supply current of the PFM
control scheme make it the better choice. The output
voltage ripple with the MAX774/MAX775/MAX776 can
be held quite low. For example, using the circuit of
Figure 2, only 100mV of output ripple is produced when
generating a -5V at 1A output from a +5V input.
Bootstrapped vs.
Non-Bootstrapped Operation
Figures 2 and 3 are the standard application circuits for
bootstrapped mode, and Figure 4 is the circuit for non-
bootstrapped mode. Since EXT is powered by OUT,
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
Inverting DC-DC Controllers
10 ______________________________________________________________________________________
MAX774
MAX775
MAX776
OUT V+
SHDN CS
FB EXT
GND
P
7
8
C2
0.1
µ F
R1
0.07
Ω
C1
150 µ F
6
5
2
3
1
REF
4
Q1
Si9435
1N5822/
MBR340
L1
22 µ H
C4 *
V IN
C3
0.1 µ F
V OUT
* MAX774 = 330 µ F, 10V
MAX775, MAX776 = 120 µ F, 20V
PRODUCT
MAX774
MAX775
MAX776
OUTPUT
VOLTAGE (V)
-5
-12
-15
INPUT
VOLTAGE (V)
3 to 15
3 to 8
3 to 5
OUTPUT
CURRENT (A)
1
0.5
0.4
NOTE: Si9435 HAS V GS OF 20V MAX
MAX774
MAX775
MAX776
OUT V+
SHDN
CS
FB
EXT
GND
P
7
8
R2 R3
0.07
Ω
C1
150 µ F
6
5
2
3
1
REF
4
Q1
Si9435
1N5822/
MBR340
L1
22 µ H
C4 *
V IN
C3
0.1 µ F
V OUT
C2
0.1
µ F
R1
* MAX774 = 330 µ F, 10V
MAX775, MAX776 = 120 µ F, 20V
Figure 2. Bootstrapped Connection Using Fixed Output
Voltages
Figure 3. Bootstrapped Connection Using External Feedback
Resistors
Figure 4. Non-Bootstrapped Operation (V IN > 4.5V)
MAX774
MAX775
MAX776
OUT V+
SHDN
CS
FB
EXT
GND
P
7
8
R2 R3
0.07
Ω
C1
150 µ F
6
5
2
3
1
REF
4
Q1
Si9435
1N5822/
MBR340
L1
22 µ H
C4 *
V IN
C3
0.1
µ F
V OUT
C2
0.1
µ F
R1
* MAX774 = 330 µ F, 10V
MAX775, MAX776 = 120 µ F, 20V