Block Diagram
V
V
C
V
CC
RG
I
C
Oscillator
Comparator
V
GATE
GATE
C
OSC
Flip-Flop
A1
7I
G1
R
Q
C
PGnd
F2
V
FB
S
Q
G2
Comparator
V
-
FB
A6
2.5V
1.5V
1.25V
+
0.7V
V
CC
+
Hold
Off
Comp
V
CC
-
-
Fault
Comp
1.15V
+
G4
CS Charge
Sense
Comparator
G3
I
A4
T
CS
Comparator
CS
2.3V
A2
R
Q
Q
F1
I
I
T
T
5
55
G5
S
1.5V
2.5V
Slow Discharge
Flip-Flop
A3
+
Slow Discharge
Comparator
2.4V
Gnd
Figure 1: Block Diagram for CS51033
Circuit Description
At startup, the voltage on all pins is zero. As VCC rises, the
VC voltage along with the internal resistor RG keeps the
PFET off. As VCC and VC continue to rise, the oscillator
capacitor (COSC ) and the Soft start/Fault Timing capacitor
(CS) charges via internal current sources. COSC gets charged
by the current source IC and CS gets charged by the IT
source combination described by:
Theory of Operation
Control Scheme
The CS51033 monitors the output voltage to determine
when to turn on the PFET. If VFB falls below the internal ref-
erence voltage of 1.25V during the oscillator’s charge cycle,
the PFET is turned on and remains on for the duration of the
charge time. The PFET gets turned off and remains off dur-
ing the oscillator’s discharge cycle time with the maximum
duty cycle to 80%. It requires 7mV typical, and 20mV maxi-
mum ripple on the VFB pin is required to operate. This
method of control does not require any loop stability com-
pensation.
IT IT
ICS = IT -
+
(
)
55
5
The internal Holdoff Comparator ensures that the external
PFET is off until VCS > 0.7V preventing the GATE flip-flop
(F2) from being set. This allows the oscillator to reach its
operating frequency before enabling the drive output. Soft
start is obtained by clamping the VFB comparator’s (A6) ref-
erence input to approximately 1/2 of the voltage at the CS
pin during startup, permitting the control loop and the out-
put voltage to slowly increase. Once the CS pin charges
above the Holdoff Comparator trip point of 0.7V, the low
Startup
The CS51033 has an externally programmable soft start fea-
ture that allows the output voltage to come up slowly, pre-
venting voltage overshoot on the output.
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
