11
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
25501.04
R6 is a damping resistor, which buffers IS terminal current
against surge currents, such as inrush currents. It is recommended
to have a value of 100 .
C3, if necessary to reduce high frequency noise, is recommended
to have a capacitance of in the range of 0.1 to 10 nF.
IS Upper Overcurrent Protection, VIS(OCPH) If the IS termi-
nal voltage decreases to VIS(OCPH) = 0.55 V (typ) or lower, the
control circuit limits the output power on a pulse-by-pulse basis
by setting both OUT1 and OUT2 low, which turns off both power
MOSFETs. This is shown in Figure 16. This protection function
operates under abnormal conditions such as when an inductor is
shorted or is saturated.
Overvoltage Protection (OVP)
The overvoltage protection function has two stages, Soft OVP,
and OVP, illustrated in Figure 17 and described below.
VFB Output Soft Overvoltage Protection, VFB(SOVP) When
VFB terminal voltage increases to VFB(SOVP) = 3.68 V (typ), Soft
Overvoltage Protection is activated. This discharges the COMP
terminal by ICOMP(SK) = 100 A (typ) and the output voltage is
decreased. VFB(SOVP) = 3.68 V (typ) is equivalent to about 105%
of the rated output voltage, VOUT
.
The output voltage threshold that initiates Soft Overvoltage Pro-
tection, is calculated approximately as follows:
VOUT(SOVP)
×
VFB(SOVP)
VOUT(norm)
VFB(REF)
(2)
where VOUT(norm) is VOUT under normal operating conditions, and
VFB(REF) is the Error Amplifier reference voltage, 3.5V (typ).
VFB Output Overvoltage Protection, VFB(OVP) When the
VFB terminal voltage increases to VFB(OVP) = 3.72 V (typ), both
OUT1 and OUT2 are set low on a pulse-by-pulse basis, which
stops the output supply by turning off the power MOSFETS.
When VFB terminal voltage decreases to VFB(SOVP)
, the control
circuit stops discharging from the COMP terminal and restores
switching operation.
The output voltage threshold that initiates Overvoltage Protec-
tion, is calculated approximately as follows:
VOUT(OVP)
×
VFB(OVP)
VOUT(norm)
VFB(REF)
(3)
where VOUT(norm) is VOUT under normal operating conditions, and
VFB(REF) is the Error Amplifier reference voltage, 3.5V (typ).
R3 is recommended to be a high-value resistor, in the range from
several hundred k to several M, ±1% tolerance, and of an
anti-electromigration type, such as metal oxide film.
C8 is recommended if necessary to reduce high frequency noise,
and should have a rating of 0.1 to 10 nF.
Open Loop Detection (OLD)
In the event that the output voltage detection resistor, R3 (see
Figure 18), opens and VFB terminal voltage decreases to
VFB(OLDL) = 0.5 V (typ) or less, the control circuit stops the
switching operation and enters standby mode. VFB(OLDL) =
0.5 V (typ) is equivalent to about 14.3% of the rated output volt-
age, VOUT .
When the VFB terminal voltage subsequently increases to
VFB(OLDH) = 0.7 V (typ) or more, the control circuit restores
switching operation. VFB(OLDH) = 0.7 V (typ) is equivalent to
about 20% of the rated output voltage, VOUT .
Figure 17. Overvoltage operation waveform and external circuit
CV OV
Time
3.50 V(typ)
VFB terminal
voltage
3.72 V(typ)
3.68 V(typ)
OUT1,OUT2 is set to low
CV : Constant voltage operation period
Time
COMP terminal
current
0
100Ǵ
A(typ )
CV
V
FB(OVP)
V
FB(SOVP)
106% of V
OUT
105% of V
OUT
V
OUT
=100%
OV : Overvoltage operation period
V
FB(REF)
I
COMP(SK)
4
%
%1/2
8+0
8($
8%%
+5
176
)0&
176
55%
4
4
% %%
Ǵ#
8
176